Article pubs.acs.org/IC
Synthesis, Structure, and Photophysical Properties of Mo2(NN)4 and Mo2(NN)2(TiPB)2, Where NN = N,N′‑Diphenylphenylpropiolamidinate and TiPB = 2,4,6-Triisopropylbenzoate Changcheng Jiang,* Philip J. Young,† Christopher B. Durr,‡ Thomas F. Spilker,§ and Malcolm H. Chisholm Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States S Supporting Information *
ABSTRACT: Two dimolybdenum compounds featuring amidinate ligands with a CC bond, Mo2(NN)4 (I), where NN = N,N′-diphenylphenylpropiolamidinate, and trans-Mo2(NN)2(TiPB)2 (II), where TiPB = 2,4,6-triisopropylbenzoate, have been prepared and structurally characterized by single-crystal X-ray crystallography. Together with Mo2(DAniF)4 (III), where DAniF = N,N′-bis(p-anisyl)formamidinate, all three compounds have been studied with steady-state UV−vis, IR, and time-resolved spectroscopy methods. I and II display intense metal to ligand charge transfer (MLCT). Singlet state (S1) lifetimes of I−III are determined to be 0.7, 19.1, and 2.0 ps, respectively. All three compounds have long-lived triplet state (T1) lifetimes around 100 μs. In femtosecond time-resolved infrared (fs-TRIR) experiments, one ν(CC) band is observed at the S1 state for I but two for II, which indicate different patterns of charge distribution. The electron would have to be localized on one NN ligand in I and partially delocalized over two NN ligands in II to account for the observations. The result is a standard showcase of excited-state mixed valence in coordination compounds.
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INTRODUCTION Charge distribution at the lowest singlet excited state (S1) of coordination compounds with metal to ligand charge transfer (MLCT) transitions is an interesting topic.1−3 One famous example is the charge distribution in the MLCT singlet state (1MLCT) of Ru(bpy)32+.1,4−6 Having a C3 axis means that the electron may be distributed equally or unequally over the three bpy ligands following photon absorption. Though its T1 state (3MLCT) was clearly revealed as a charge-localized state with all electron density on a single bpy ligand,7 the nature and electrodynamics of its S1 state is not very well understood and has led to intense debates.6,8,9 The ultrafast intersystem crossing (ISC, less than 50 fs) has been a major difficulty in examining the S1 state. DeArmond once described the π* ligand orbitals in such metal-chelated compounds as “spatially isolated orbitals”, stressing the weak interactions between them, based on evidence such as negative charge localization in corresponding one-electron-reduced species and dual phosphorescence from mixed-ligand compounds.10 We believe, however, that the strength of communication or electronic coupling between the ligand orbitals varies substantially due to metal−ligand © XXXX American Chemical Society
interactions and electronic structure details in an individual compound. We have studied the excited states of paddlewheel compounds of the forms Mo2(O2CR)4 and Mo2(O2CR)2(TiPB)2, where R = alkenyl, alkynyl, aryl and TiPB = 2,4,6-triisopropylbenzoate, using femtosecond transient absorption spectroscopy (fs-TA) and femtosecond timeresolved infrared spectroscopy (fs-TRIR).11−15 These compounds have intense light absorption in the UV−vis spectrum due to metal (Mo2δ) to ligand π* charge transfer (MLCT) transitions. The transition typically generates a chargeseparated, metal to ligand charge transfer singlet state (1MLCT) that subsequently undergoes intersystem crossing (ISC) to a metal-centered triplet state (3MoMoδδ*). The 1 MLCT S1 lifetimes are on the order of 1−20 ps and 3 MoMoδδ* T1 lifetimes are on the order of 10−100 μs.16 T y p i c a l l y , i n t h e c o m p o u n d s M o 2 ( O 2 C R) 4 a n d Mo2(O2CR)2(TiPB)2, where R = −C4H2SCCH, −C6H4-pReceived: January 15, 2016
A
DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry CCH, extensive charge delocalization (between ligands that are trans to each other) in the 1MLCT state was found.15 In some ways these charge-separated excited states can be compared to mixed-valence compounds and classified in the same spirit.17 As proposed by Zink, a class I (similar to the Robin−Day scheme) MLCT excited state is one where just a single ligand possesses the negative charge, a class III MLCT excited state is one in which the charge is completely delocalized, and a class II MLCT excited state is defined as an intermediate, where the charge is localized but has a low barrier to interconversion. Thus, we assigned the aforementioned localized 3MLCT T1 state of Ru(bpy)2+ as class I while assigning the delocalized 1MLCT S1 states of the aforementioned Mo2(O2CR)4 and Mo2(O2CR)2(TiPB)2 compounds as class III. Yet, there was one such amidinate compound that was examined previously, Mo2[(iPrN)2CCCPh]2(O2CMe)2, in which we discovered that the charge is primarily localized on one ligand but with some charge spillover to the other ligand.18 Thus, its S1 state was classified as a class II excited state. This class II amidinate compound prompted us to look at similar compounds and try to understand how ground-state electronic structure influences the charge distribution in the excited state. In this report, analogous compounds bearing amidinate ligands of the form Mo 2 (N 2 CR) 4 and Mo2(N2CR)2(TiPB)2 have been synthesized and photophysically characterized. Interestingly, different charge distribution patterns are observed with I and II, which have the same MLCT-active ligand.
Mo2(OAc)4 + 4Li+(NN)− THF
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Mo2(NN)4 + 4Li+(OAc)− 50 ° C, 3 days
(1)
Mo2(TiPB)4 + 2Li+(NN)− THF
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ trans‐Mo2(NN)2 (TiPB)2 + 2Li+(TiPB)− room temp, 1 day
(2)
Compound III was prepared by a routine method described previously.20 All compounds were purified by solution methods and characterized by 1H NMR and MALDI-TOF to ensure purity. The detailed synthesis can be found in the Experimental Section, and the NMR and MALDI-TOF mass spectra are included in the Supporting Information. Single crystals suitable for X-ray analysis of I and II were grown by vaper diffusion of hexanes into concentrated THF solution with a small amount of DMSO. Single-Crystal and Molecular Structure Determination. The structure of Mo2(DAniF)4 (III) was previously determined by Cotton and co-workers.20,21 The essential feature of the structure is the basic Mo2[NC(H)N]4 unit having near D4h symmetry. The anisole groups on the nitrogen are twisted from the central NC(H)N plane and therefore have no effective low-energy charge-transfer transitions. This is similar to the case for dimolybdenum tetraformate, Mo2(O2CH)4. However, for compounds I and II, the presence of the conjugating −CCPh unit reduces the energy of the transition. We examined the crystal of I by single-crystal X-ray diffraction studies. The structure is shown in Figure 2. The
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RESULTS AND DISCUSSION Two dimolybdenum amidinate compounds with IR-active C C bonds (I and II) are studied here. Their electronic structure and excited-state spectra are discussed. One prototypical Mo2 amidinate compound (III) is studied here as well. All three molecules are shown in Figure 1.
Figure 2. Two views of the crystal structure of I. Thermal ellipsoids are drawn at the 50% probability level. Hydrogens have been removed for clarity. Color code: teal, molybdenum; blue, nitrogen; gray, carbon.
central Mo2(NCN)4 units had virtual D4h symmetry, as expected for a homoleptic Mo2 quadruply bonded compound. The Mo−Mo distance is 2.10 Å, and the average Mo−N distance is 2.14 Å. Coordination of the NN ligands to Mo2 center results in a slight twisting; the average torsion angle is 8.2°. The twist is likely to be caused by crowded phenyl groups on the nitrogen atom. In this configuration there is still extensive conjugation in the PhCCCN2−Mo2−N2CCCPh chain. The distance from one terminal carbon to the other terminal carbon is ∼19 Å. The X-ray crystal structure of II is shown in Figure 3. Here there is a central trans-Mo2(NCN)2(OCO)2 unit with near-D2h symmetry except that one Mo atom has a coordinating DMSO solvent. The aryl groups in TiPB are twisted by 90 and 80° relative to the CO2 plane, thus removing them from conjugating with the Mo2δ bond. The amidinate ligands,
Figure 1. Structures of I−III. Aryl groups on nitrogen are abbreviated for clarity.
Synthesis. I and II were prepared by displacing carboxylate ligands in dimolybdenum carboxylates with lithium N,N′diphenylphenylpropiolamidinate. Dimolybdenum tetraacetate, Mo2(OAc)4, was used as the precursor for compound I, and Mo2(TiPB)4 was used for II. Hicks et al. reported the synthesis of I previously, and here the same procedure was used.19 The reactions are shown in eqs 1 and 2. B
DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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HOMO and merely ∼1% to the LUMO. The results are summarized in Table S2 in the Supporting Information. From the frontier MO diagram shown in Figure 4, we see that with the introduction of the amidinate ligands the energy
Figure 3. Two views of the crystal structure of II. Coordination of a DMSO solvent molecule (at the back) is shown on the left, and the twist of aryl moieties on TiPB ligands is shown on the right. Thermal ellipsoids are drawn at the 50% probability level. Hydrogens have been removed for clarity. Color code: teal, molybdenum; blue, nitrogen; red, oxygen; yellow, sulfur; gray, carbon.
however, form a nearly flat plane with the Mo2 unit. The torsion angles between the two phenyl rings and their respective NCN planes are 10 and 11°. The PhCCCN2− Mo2−N2CCCPh chain is slightly tilted, as has been seen for many molecules with extended (−CC−)n units.22 Due to the tilting, the terminal carbon atoms lie ∼2 Å below the Mo2(NCN)2 plane. The DMSO molecule is found to coordinate to the Mo2 center in II through a Mo−O bond on the axial site. The Mo− O distance is ∼2.45 Å, which is typical for a solvent molecule bound to the Mo2 unit. However, this makes the Mo−N distances for the molybdenum atom with the coordinating solvent molecule slightly longer in comparison to the Mo atom that does not. The Mo−Mo distance is 2.11 Å, and the average Mo−N distance is 2.13 Å. Electronic Structure Calculations. Electronic structure calculations of I and II were carried out using density functional theory (DFT) in Gaussian 09.23 To show the influence of substituting carboxylate ligands with amidinate ligands, the electronic structure of a carboxylate compound, Mo2(O2CC CPh)4 (referred to as Mo2(OO)4 later), was also investigated. To simplify the calculations, phenyl groups on the nitrogen atoms in I and II and aryl groups of TiPB in II were replaced by hydrogen atoms. The introduction of four −CCPh units in I, which are in conjunction with the C(NH)2 units, give rise to four π*acceptor orbitals (eu, b2g, and a1g). The b2g and a1g π* orbitals are higher in energy because these vacant orbitals are open to back-bonding from the Mo2 center. This interaction leaves the eu orbitals as the lowest π* orbitals, lying between the Mo2δ (b2g) and Mo2δ* (b1u) orbitals. Similar to the case for Mo2(OO)4, the resulting HOMO of I is mainly composed of the Mo2δ bond with some b2g ligand π* orbital character; the LUMO is composed of ligand eu π* orbitals. In II, the symmetry is reduced and combinations of the two NN ligands form two π* orbitals (b1u and b1g) with one where two ligand orbitals are in phase (b1u) and the other out of phase (b1g). Here, again, the energy of the b1g combination is raised by the Mo2δ back-bonding, which leaves the b1u combination lower in energy. The transition from HOMO to LUMO in both molecules is fully allowed. Natural bonding orbital (NBO) population analyses were carried out for the three compounds.24 The contribution of the metal orbital to the LUMO in all three compounds is small. The dimolybdenum center contributes more than 50% to the
Figure 4. Molecular orbital (MO) energy diagram of I, II, and Mo2(OO)4. Filled orbitals are shown in blue, and vacant orbitals are shown in red. Four selected MO isosurfaces of compound I (LUMO +4 (with δ* character), LUMO/LUMO+1 (degenerate), and HOMO) are shown on the right.
of the Mo2δ orbital rises significantly in comparison to the Mo2(OO)4 analogue. The energy of the Mo2δ orbital increases by ∼0.9 eV in I and ∼0.3 eV in II. However, the rises of both the HOMO and LUMO are similar; thus, similar energies for electronic transitions are expected. One interesting scenario shared by I and Mo2(OO)4 is that the electron is excited into a degenerate eu orbital in the MLCT transition. As mentioned in a previous study,15 the MLCT excited state is subjected to the Jahn−Teller effect and splitting of the two eu orbitals is expected. Infrared active vibrational frequency analyses were also carried out on I and II for the ground state and triplet state to help explain the spectra from the fs-TRIR experiments. Reliable calculations of the S1 state of coordination compounds with heavy metals are complicated and computationally expensive. Here the calculations of molecular anions (by putting a negative charge onto the neutral molecule) were used to mimic the charge-separated S1 state. Because the standard setting of the B3LYP functional is known to always overestimate exchange in mixed valence systems,25 the results can only be used as a rough reference. These will be discussed in TimeResolved Infrared Spectra. Electronic Absorption. UV−vis absorption spectra of I− III were collected in THF solution at room temperature. The spectra are shown in Figure 5. I and II show similar, intense MLCT absorptions, centered at 521 and 500 nm, respectively. The molar extinction coefficient (ε) of the MLCT band for I is 46000 M−1 cm−1, roughly 2 times that of II, 26000 M−1 cm−1. This is justified, as I has twice as many NN ligands as II. The absorption centered around 280 nm in I and II is attributed to ligand-centered transitions (LLππ*). The absorption band centered at 335 nm in I is attributed to a high-energy MLCT from a metal−metal π orbital to a ligand π* orbital and is not seen in II. The MLCT transition for III appears around 430 nm. Here, without conjugated accepting groups, the MLCT transition is rather weak in comparison to its LLππ* transitions around 300 nm. C
DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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results are summarized in Table 1. For all three compounds, the S1 states are assigned as 1MLCT states and T1 states are assigned as 3MoMoδδ* states on the basis of their lifetimes and fs-TRIR spectra. Table 1. Lifetimes of I−IIIa τ(S1), ps I II III a
Figure 5. UV−visible spectra of I (red), II (yellow), and III (gray), taken in THF solution at room temperature. Absorption is plotted as the molar extinction coefficient, ε.
0.7 19.1 2.0
S1 state 1
MLCT 1 MLCT 1 MLCT
τ(T1), μs 92 113 81
T1 state 3
MoMoδδ* 3 MoMoδδ* 3 MoMoδδ*
The S1 and T1 states are assigned.
The fs-TA spectra for I−III were taken in THF at room temperature. The excitation wavelength is 515 nm for I and II, and 350 nm for III. Features from 350 to 700 nm were collected. Due to pump laser interference, data points around 515 nm for I and II were excluded. The fs-TA spectra of I are shown in Figure 7. A clear transient absorption peak at ∼420 nm is assigned to the
Emission Spectroscopy. Emission spectra for I−III are shown in Figure 6, which were taken in THF solution at room
Figure 6. Normalized fluorescence (solid lines) and phosphorescence (dashed lines) of I (red), II (orange), and III (gray). Spectra were taken in THF solution at room temperature. Fluorescence of III is not observed.
Figure 7. fs-TA spectra of I taken in THF at room temperature, with λex 515 nm. 1
MLCT S1 state. There is a strong, broad bleach from 430 to 550 nm due to loss of ground-state absorption. Over time, a rapid decay of the 1MLCT S1 state absorption is observed, leaving a strong 3MoMoδδ* T1 state absorption peak centered at ∼370 nm. The fs-TA spectra of II are shown in Figure 8. The 1MLCT S1 state transient absorption of II is quite similar to that of I, which is centered at 430 nm. Interestingly, we also see stimulated emission of II from 520 to 650 nm and the kinetics of emission matches with the 1MLCT S1 absorption. At longer
temperature. I and II show both fluorescence and phosphorescence. The fluorescence for I is very weak, centered at 580 nm, and only a portion of the spectrum is shown due to scattering from solvent. II has a relatively strong fluorescence centered at 556 nm. The fluorescence quantum yield for II was determined to be ∼0.3%. Phosphorescence is observed at 895 nm for I and 910 nm for II. No fluorescence is observed for III, but phosphorescence is seen at 875 nm. The observed phosphorescence of these amidinate compounds is in contrast with the phosphorescences seen for Mo2(O2CR)4 compounds that appear close to 1100 nm. The reason may be that the amidinate compounds tend to have smaller Mo−Mo bond length changes in the T1 state in comparison to carboxylate compounds. Hopkins et al. had shown that the T1 state energy is strongly related to the Mo− Mo distance.26 This explanation is also supported by previous ionization studies which shows that dimolybdenum amidinates have smaller Mo−Mo bond length changes in comparison to carboxylates upon removing electrons from the δ bond.20 Transient Absorption Spectra. All three compounds were examined by femtosecond transient absorption spectroscopy (fs-TA) and nanosecond transient absorption spectroscopy (nsTA). Similar excited state features were seen in the three compounds. The S1 lifetimes and T1 lifetimes were extracted from fs-TA spectra and ns-TA spectra, respectively. The fitted S1 lifetimes were cross-examined by fs-TRIR experiments. The
Figure 8. fs-TA spectra of II taken in THF at room temperature, with λex 515 nm. D
DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 9. Kinetic traces for S1 state absorptions of I (left) and II (right). The traces were taken at 410 and 420 nm from the fs-TA spectra of I and II, respectively.
shows strong bands at 1488 and 1445 cm−1 due to CO2 and CN2 stretches, ν(O−C−O) and ν(N−C−N), respectively. Computational methods were used to aid the interpretation of fs-TRIR spectra. In particular, infrared frequency analyses of molecular anions of I and II, i.e. I− and II−, were used to simulate the 1MLCT states of I and II where an extra electron is added to the ligands. The triplet state forms of I and II, 3I and 3II, were calculated to investigate the 3MoMoδδ* state. These results are shown in Figures S11 and S12 in the Supporting Information. The calculations of I‑ predicted that the ν(CC) band in I would shift to ∼75 cm−1 to lower energy relative to the ground state in the 1MLCT state and split into two peaks (2150 and 2120 cm−1, a Jahn−Teller scenario). The calculations of II− predicted that the ν(CC) band in II would shift 135 cm−1 to lower energy. Considering that the electron is distributed evenly over all available NN ligands in I− and II−, the shift would be around 200−300 cm−1 when the charge is localized on one ligand. The 3I and 3II calculations predicted that the ν(CC) band in the 3MoMoδδ* T1 state would reside close to the ground state, ∼5 cm−1 higher in energy. The fs-TRIR spectra of I are shown in Figure 10. Here the spectra are divided into three regions corresponding to the
time delays, the 3MoMoδδ* T1 state of II shows an absorption peak at 380 nm and also shows a broad absorption feature from 540 to 650 nm. The fs-TA spectra of III are shown in Figure S6 in the Supporting Information. III has a clear S1 state absorption band centered at 470 nm which fades to give a broad T1 absorption band centered at a similar value. The kinetic plots of I and II are shown in Figure 9. Both kinetic traces are best fitted with a biexponential function. The two lifetime components τ1 and τ2 are shown with percent contribution. In I, the lifetime of the S1 state is determined to be 0.7 ps. The longer τ2 value cannot be the S1 state lifetime because it does not agree with the fs-TRIR experiment. τ2 fluctuated around 10 ps when traces were taken at different wavelengths. This component could be attributed to a slow charge-hopping process between ligands.6 In II, the S1 lifetime is determined to be 19.1 ps, which again agrees with the results from the fs-TRIR section. The 0.4 ps component is attributed to cooling processes. The S1 lifetime of I is over 1 order of magnitude shorter than that of II. Considering that the two compounds share the same active ligand, this is quite curious. All three compounds were also examined by nanosecond transient absorption spectroscopy (ns-TA) to study the triplet state. The spectra are given in Figures S8−S10 in the Supporting Information. The same T1 state features of I−III are observed in the ns-TA spectra as in the fs-TA spectra at long time delays. T1 lifetimes were determined from the transient features in ns-TA spectra, and the kinetic plots are shown within the ns-TA spectra. It is quite obvious that T1 lifetimes of these three dimolybdenum amidinates are longer than those of dimolybdenum carboxylates. The main reason could be that these compounds have higher T1 state energy levels; thus, vibrational relaxation is discouraged. Time-Resolved Infrared Spectra. The excited states of I and II were studied by femtosecond time-resolved infrared spectroscopy (fs-TRIR) in THF at room temperature. Generally, features from 2250 to 1400 cm−1 were collected and can be attributed to three different vibrations: the CC stretch ν(CC), the phenyl ring stretch ν(C6)ring in the N2C− CC−Ph moiety, and the CN2 and CO2 stretches ν(N−C− N)/ν(O−C−O). Differences in charge distribution in the MLCT states in I and II are determined from the ν(CC) shift pattern. Ground-state infrared spectra (gs IR) of I and II were collected with a Fourier transform IR spectrometer. Both I and II show a very weak ν(CC) band around 2210 cm−1 and a moderate ν(C6)ring band around 1595 cm−1. I shows a strong band at 1465 cm−1 due to CN2 stretches ν(N−C−N), and II
Figure 10. Femtosecond TRIR spectra of I taken in THF at room temperature, with λex 515 nm. Time delays are shown in the inset. The vertical dashed lines divide spectra into specific sections.
three IR active vibrations: ν(CC), ν(C6)ring, and ν(N−C− N). The ground-state IR (gs IR) is plotted as a dotted line. Upon photoexcitation, I shows a broad band centered at 1965 cm−1 at short time delays. This peak is assigned to ν(C C), which shifts significantly (245 cm−1) from the value for the ground state and fades away completely in picoseconds. This agrees with its assignment to the 1MLCT S1 state. At lower energy we see a weak dip at 1594 cm−1 due to bleaching of the ring stretch ν(C6)ring. The lifetime of the S1 state was E
DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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lifetimes of I and II are determined to be 0.53 and 18.9 ps, respectively; the results are close to those extracted from the fsTA spectra. The appearance of a single ν(CC) band and the shift of the band 245 cm−1 to lower energy are indicative of a singly reduced ligand in the 1MLCT S1 state for I. The magnitude of this shift is very similar to the 240 cm−1 shift that is seen for the compound (DAniF)3Mo2(O2CCCPh), which only has one alkynyl group, and the charge has to be localized.27 In II, the appearance of two shifted ν(CC) bands (70, 235 cm−1) is akin to that for the previously studied compound Mo2[(iPrN)2CCCPh]2(O2CMe)2 (45, 241 cm−1).18 We believe the ν(CC) band shifted by 235 cm−1 in II indicates that the charge mainly resides on one alkynyl group, similar to the case for I. However, the second band which shifted by 70 cm−1 indicates that the other alkynyl group is also weakened in the S1 state. We assigned this as a partially delocalized case. It is noted that the ν(CC) band in I is not very symmetric and remarkably broad. There appears to be a shoulder peak at ∼2030 cm−1 lying on the high-energy side of the ν(CC) band. We have yet to determine an appropriate explanation for this shoulder, only that the ν(CC) band in (DAniF)3Mo2(O2CCCPh) is similarly unsymmetrical and broad.27 The observed ν(CC) patterns in the 1MLCT states of I and II do not fit with the pattern of previously studied homoleptic Mo2L4 compounds and heteroleptic trans-Mo2L2L′2 compounds. In the case of homoleptic Mo2(O2CC6H4-p-C N)4 and Mo2(O2CC6H4-p-CCH)4, similar ν(CC) and ν(CN) patterns and band shifts are observed in Mo2L4 with corresponding trans-Mo2L2(TiPB)2 compounds.3,15 We concluded that these are special delocalized cases (class III) and only one set of trans ligands is involved in the 1MLCT state of Mo2L4 compounds. This is, obviously, not the case with I. The absence of the ν(CC) band shifted by 70 cm−1 in I indicates that only one NN ligand is involved in the 1MLCT state. Thus, we assigned the 1MLCT S1 state in I as a class I excited state and the 1MLCT S1 state in II as a class II excited state. The reason II has a class II 1MLCT S1 state while I has a class I 1MLCT S1 state is not quite clear yet. In a previous review, we suggested a donor−bridge−acceptor model to rationalize the excited state mixed valence.28 In that model, one of the ligands acts as a donor, the other acts as an acceptor, and the Mo2 center acts as the bridge. In classic mixed-valence compounds, bridges usually play a crucial role in mediating electron transfer. We think that, on comparison of I and II, the energy levels of the Mo2δ and Mo2δ* orbitals of I are raised much higher than those of II, and the high-energy Mo2δ* orbital of I may discourage the electron transfer in the S1 state. Another possible explanation is that the distortion induced by the Jahn−Teller effect in I changed the vibronic interactions in
determined to be 0.53 ps, which is close to the lifetime extracted from previous fs-TA spectra. At longer time delays, we observed a strong ν(N−C−N) bleach at 1456 cm−1 and an associated strong absorption feature grows at 1480 cm−1. The CN2 stretch moves to high energy because less electron density would donate to the CN2 π* orbital from the Mo2δ bond in the 3 MoMoδδ* T1 state in comparison to the ground state. Also at higher energy, we see a weak peak at 2205 cm−1 due to ν(C C) in the T1 state, shifted ∼5 cm−1 to lower energy from the ground state (rather than to higher energy, as predicted from calculations). The fs-TRIR spectra of II are shown in Figure 11. Here again, the spectra are divided into three sections and the gs IR is plotted as a dotted line.
Figure 11. Femtosecond TRIR spectra of II taken in THF at room temperature, with λex 515 nm. Time delays are shown in the inset. The vertical dashed lines divide spectra into specific sections.
At short time delays, we see two bands instead of one corresponding to ν(CC): one weaker band at 2140 cm−1 and one stronger band at 1975 cm−1 which are shifted 70 and 235 cm−1 from the values for the ground state, respectively. Both features lasted much longer than the ν(CC) feature seen in I, and there is no obvious peak shifting during the course. The lifetime of these ν(CC) features is determined to be 18.9 ps, which agrees well with the fs-TA experiment. In addition, at short time delays, a similar bleach at 1595 cm−1 and a sharp absorption peak at 1558 cm−1 are observed for ν(C6)ring. The absorption of ν(C6)ring is shifted 37 cm−1 to lower energy in comparison to the peak for the ground state. This sharp ν(C6)ring absorption is not observed in I. At longer delays, the strong bleach at 1489 cm−1 and absorption at 1510 cm−1 grows in and these are assigned to ν(O−C−O). Kinetic plots for the decay of fs-TRIR features of I and II are shown in Figure 12. The decays of ν(CC) bands in I are best fitted with a monoexponential function, while traces of II are best fitted with a biexponential function. The 1MLCT S1 state
Figure 12. Kinetic traces for fs-TRIR spectra of I (left) and II (right), taken at 1965 and 1975 cm−1, respectively. F
DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
global minima on the potential surface. Isosurface contour plots were created with Gaussview 5.0.8 with isovalues at 0.02. Vibrational frequency results were scaled by a factor of 0.961 as suggested. Steady-State Absorption and Emission Experiments. Steadystate electronic absorption and emission measurements were carried out with a 1.0 × 1.0 cm quartz cuvette equipped with a Kontes top. Electronic absorption spectra were recorded using a PerkinElmer Lambda 900 spectrometer at room temperature. Molecular extinction coefficients were determined with four samples at different concentrations. Emission in the UV−vis range was determined on a SPEX Fluoromax-2 fluorometer. Emission in the near-infrared (NIR) region (800−1200 nm) was determined on a home-built instrument equipped with a germanium detector. FT-IR Spectra. Ground-state IR spectra from 1200 to 4000 cm−1 were collected on a PerkinElmer Spectrum GX FTIR spectrometer. Sample solutions were sealed in a PerkinElmer semidemountable cell with CaF2 windows and a 0.1 mm Teflon spacer. Nanosecond Transient Absorption Experiments. Nanosecond transient absorption spectra were collected on a home-built pumpedprobe instrument which was equipped with a Spectra-Physics GCR150 laser source (Nd:YAG laser, fwhm ∼10 ns) and a flash lamp. Transient signals were collected with a Hamamatsu R928 photomultiplier tube and were processed with a Tektronics 400 MHz oscilloscope (TDS 380). Sample solutions were hold in a 1.0 × 1.0 cm square quartz cuvette with a Kontes top. Femtosecond Time-Resolved Experiments. Femtosecond transient absorption (fs-TA) and femtosecond time-resolved infrared (fs-TRIR) experiments were performed using a Ti:sapphire oscillator/ regenerative amplifier combination (1 kHz, fwhm ∼300 fs), as previously described.35,36 In fs-TA experiments, the excitation wavelength was 515 nm for I and II and 350 nm for III. Samples were prepared to have 0.2−0.4 absorption at the excitation wavelength. The excitation power at the sample was tuned to 1−2 μJ. The spectra collected underwent wavelength calibration and GVD corrections. In the fs-TRIR experiments, the same excitation wavelength was used as for the fs-TA experiments. Samples of I and II were prepared to have ∼1 absorption at the MLCT peaks. Sample solutions were kept air-free in a PerkinElmer semidemountable cell with a 0.1 mm Teflon spacer and two 4.0 mm CaF2 windows. All time-resolved spectra were plotted in Igor Pro 6.0. Kinetic traces were fitted with a global fitting package with a sum of exponentials, S(t) = ∑iAi exp(−1/τi) + C, where Ai is the amplitude, τi is the lifetime, and C is an offset. Preparation of Lithium N,N′-diphenylphenylpropiolamidinate. Lithium N,N′-diphenylphenylpropiolamidinate was obtained by reacting (phenylacetyl)lithium with N,N′-diphenylcarbodiimide. N,N′Diphenylcarbodiimide was prepared by refluxing N,N′-diphenylthiourea (4.0 g, 8.7 mmol, 1 equiv) with HgO (5.6 g, 13.0 mmol, 1.5 equiv) and dry CaCl2 (3.0 g) in 20 mL of acetone for 1 h. The mixture was then filtered twice and the solvent removed to leave a clear, oil-like liquid. (N,N′-Diphenylcarbodiimide is moisture sensitive and tends to polymerize but can be stored in a refrigerator for 1 week.) (Phenylacetyl)lithium was generated in situ by reacting phenylacetylene (1.0 mL, 9.8 mmol) with nBu-Li (4.0 mL, 2.5 M in hexanes, 10.0 mmol) at −78 °C. A nBu-Li solution was added dropwise, and the mixture was stirred for 30 min. N,N′-Diphenylcarbodiimide (1.9 g, 9.8 mmol) then was dissolved in 10 mL of THF in a separate Schlenk flask and cannulated into a (phenylacetyl)lithium solution. The mixture turned deep yellow and was warmed slowly and stirred overnight. The solvent was then removed in vacuo, and the remaining orange solid was redissolved in 40 mL of THF to give a 0.25 M solution of lithium N,N′-diphenylphenylpropiolamidinate. Preparation of Mo2(NN)4 (I). Mo2(OAc)4 (0.15 g, 0.35 mmol, 1 equiv) was dissolved in 20 mL of THF in a Schlenk flask. Lithium N,N′-diphenylphenylpropiolamidinate solution (6.4 mL, 1.6 mmol, 4.5 equiv) was added dropwise. The mixture turned red and was then heated to 50 °C and kept at this temperature for 3 days. The solvent was then removed in vacuo, the remaining solid was extracted with toluene, and the extract was filtered. The solvent was then removed and washed with methanol to yield a red powder. NMR (CDCl3, 400
the MLCT state. Such effects are not present in II ,and this may create a different coupling pattern. This requires further investigation.
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CONCLUDING REMARKS Electronic coupling between ligand orbitals in the excited states have been studied here by following ν(CC) in dimolybdenum amidinate compounds. In comparison to carboxylate compounds previously studied by our group, both the HOMO (Mo2δ) and LUMO (ligand π*) levels in the amidinate compounds are raised quite significantly in energy. Femtosecond TRIR spectra of I show that ν(CC) is shifted 245 cm−1 to lower energy in the 1MLCT S1 state in comparison to the ground state. The magnitude of the shift indicates that the transferred electron is localized on one ligand in the S1 state. The TRIR spectra of compound II show two ν(CC) bands in the 1MLCT S1 state, shifted by 70 and 235 cm−1 to lower energy. The two bands indicate that the electron is present on both ligands in the S1 state but is mainly localized on one. In the spirit of mixed-valence compounds, I is assigned a class I S1 state and II is assigned a class II S1 state. The much shorter S1 state lifetime of I is another interesting observation and should be a result of a faster ISC process in comparison to II. The observation is interesting in answering the question why normally M2 (where M = Mo, W) paddlewheel compounds have 1MLCT lifetimes in the picosecond domain, while 1MLCT state of complexes like Ru(bpy)32+ are much shorter lived.
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EXPERIMENTAL SECTION
General Procedures. I−III are air-sensitive compounds. Preparations and purifications of the three compounds were performed using standard Schlenk techniques under an argon atmosphere. All solvents were dried over drying agents, degassed, and stored over molecular sieves in Kontes flasks. THF used in all spectroscopy tests was dried over NaK. Mo2(Ac)4, Mo2(TiPB)4, and Mo2(DAniF)4 were prepared and purified by established methods.20,29 Sample manipulations were performed in a nitrogen-filled glovebox. NMR spectra were recorded on a 400 MHz Bruker DPX Advance 400 spectrometer. All 1H NMR chemical shifts are in ppm relative to the protio impurity in chloroform-d at 7.26 ppm. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were obtained on a Bruker Microflex mass spectrometer. Dithranol was used as the matrix, and nine peptides with molecular masses ranging from 450 to 3100 Da were used to calibrate the machine. Crystallographic Information. Crystals of I and II were isolated as dark red and orange plates and handled while covered with fluorinated oil. Diffraction data were taken on a Nonius Kappa Apex II CCD diffractometer with Mo Kα radiation. All work was done at 150 K using an Oxford Cryosystems Cryostream Cooler. The data were collected using the program APEX2 and processed using the program SAINT within APEX2. The data were scaled and absorption and beam corrections were made on the basis of the multiscan technique as implemented in SADABS.30 The structure was solved by the direct methods procedure in SHELXT.31 Full-matrix least-squares refinements based on F2 were performed in SHELXL-2014/7, as incorporated in the WinGX package.32 X-ray crystallographic data of compounds I and II in CIF format are included in the Supporting Information. Electronic Structure Calculations. The structures of simplified I and II were optimized in the gas phase using density functional theory (DFT) with the Gaussian09 suite of programs.23 The B3LYP functional was used with SDD energy consistent pseudopotentials and the SDD energy consistent basis set for Mo and 6-31G* for C, H, O, and N.33,34 Force constant and vibrational frequency analyses were performed on all structures to ensure the structure was optimized to G
DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry MHz): δ 7.08−7.20 (m, 12H), 6.95−7.05 (m, 24H), 6.86−6.92 (m, 8H), 6.16−6.22 (m, 16H). MALDI-TOF: m/z calcd 1373.31, found 1373.75. Preparation of Mo2(NN)2(TiPB)2 (II). Mo2(TiPB)4 (0.20 g, 0.17 mmol, 1 equiv) was placed in a Schlenk flask and dissolved in 20 mL of THF. Lithium N,N′-diphenylphenylpropiolamidinate solution (1.4 mL, 0.34 mmol, 2 equiv) was added dropwise. The orange solution was then stirred at room temperature for 1 day. The solvent was then removed in vacuo, the remaining solid was extracted with toluene, and the extract was filtered. The filtrate was then dried and washed with methanol to yield a yellow powder. NMR (CDCl3, 400 MHz): δ 7.15−7.30 (m, 24H), 7.06 (s, 4H), 7.00−7.08 (m, 6H), 3.35 (sep, 4H, J = 6.8 Hz), 2.91 (sep, 2H, J = 6.9 Hz), 1.31 (d, 24H, J = 6.8 Hz), 1.26 (d, 12H, J = 6.9 Hz). MALDI-TOF: m/z calcd 1277.40, found 1278.40.
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(8) Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C. V.; McCusker, J. K. Science 1997, 275, 54−57. (9) Bhasikuttan, A. C.; Suzuki, M.; Nakashima, S.; Okada, T. J. Am. Chem. Soc. 2002, 124, 8398−8405. (10) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. Rev. 1985, 64, 65−81. (11) Byrnes, M. J.; Chisholm, M. H.; Gallucci, J. A.; Liu, Y.; Ramnauth, R.; Turro, C. J. Am. Chem. Soc. 2005, 127, 17343−17352. (12) Alberding, B. G.; Chisholm, M. H.; Gustafson, T. L. Inorg. Chem. 2012, 51, 491−498. (13) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Spilker, T. F.; Young, P. J. Chem. Sci. 2015, 6, 1780−1791. (14) Alberding, B. G.; Brown-Xu, S. E.; Chisholm, M. H.; Gustafson, T. L.; Reed, C. R.; Naseri, V. Dalton Trans. 2012, 41, 13097. (15) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Spilker, T. F. J. Phys. Chem. A 2013, 117, 13893−13898. (16) Chisholm, M. H.; Gustafson, T. L.; Turro, C. Acc. Chem. Res. 2013, 46 (2), 529−538. (17) Plummer, E. A.; Zink, J. I. Inorg. Chem. 2006, 45, 6556−6558. (18) Alberding, B. G.; Chisholm, M. H.; Gallucci, J. C.; Ghosh, Y.; Gustafson, T. L. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 8152−8156. (19) Hicks, J.; Ring, S. P.; Patmore, N. J. Dalton Trans. 2012, 41, 6641. (20) Multiple bonds between metal atoms, 3rd ed.; Cotton, F. A., Murillo, C. A., Eds.; Springer Science and Business Media: New York, NY, 2005. (21) Lin, C.; Protasiewicz, J. D.; Smith, E. T.; Ren, T. J. Chem. Soc., Chem. Commun. 1995, No. 22, 2257. (22) Stahl, J.; Bohling, J. C.; Bauer, E. B.; Peters, T. B.; Mohr, W.; Martín-Alvarez, J. M.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2002, 41, 1871. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc., Wallingford, CT, 2009. (24) Lu, T.; Chen, F. J. Comput. Chem. 2012, 33, 580−592. (25) Renz, M.; Kaupp, M. J. Phys. Chem. A 2012, 116, 10629−10637. (26) Hopkins, M. D.; Gray, H. B.; Miskowski, V. M. Polyhedron 1987, 6, 705−714. (27) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Lewis, S. A.; Spilker, T. F.; Young, P. J. Inorg. Chem. 2014, 53, 637−644. (28) Chisholm, M. H.; Lear, B. J. Chem. Soc. Rev. 2011, 40, 5254. (29) Cotton, F. A.; Daniels, L. M.; Hillard, E. A.; Murillo, C. A. Inorg. Chem. 2002, 41, 1639−1644. (30) Sheldrick, G. M. SADABS, v2014/5, semi-empirical absorption and beam correction program; University of Göttingen, Göttingen, Germany. (31) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (32) Farrugia, L. J. J. Appl. Crystallogr. 2012, 45, 849−854. (33) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098− 3100. (34) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theor. Chim. Acta 1990, 77, 123−141. (35) Burdzinski, G.; Hackett, J. C.; Wang, J.; Gustafson, T. L.; Hadad, C. M.; Platz, M. S. J. Am. Chem. Soc. 2006, 128, 13402−13411.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00096. Crystal structure refinement information for I and II, NBO analysis, nanosecond TA spectra, and NMR and MALDI-TOF mass spectra of I−III (PDF) Crystallographic data for I and II (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for C.J.:
[email protected]. Present Addresses †
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. ‡ Department of Chemistry, The College of Wooster, E. University Street, Wooster, OH 44691, USA. § Department of Macromolecular Science and Engineering, Case Western Reserve University, 2080 Adelbert Road, Cleveland, OH 44106, USA. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Science Foundation for the funding associated with grant numbers CHE-0957191 and CHE1266298. We are grateful to the Ohio State University Center for Chemical and Biophysical Dynamics for use of the laser systems, the Ohio Supercomputer Center for computational resources, Dr. Judith Gallucci for help with X-ray crystallography, and Professor Claudia Turro for use of instrumentation.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b00096 Inorg. Chem. XXXX, XXX, XXX−XXX
