Metal–Metal Quadruple Bonds (M = Mo or W) Supported by 4-[2-(4

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Metal−Metal Quadruple Bonds (M = Mo or W) Supported by 4‑[2-(4Pyridinyl)ethenyl] Benzoates and their Complexes with Tris(pentafluorophenyl)boron Malcolm H. Chisholm* and Christopher J. Ziehm Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210-1185, United States S Supporting Information *

ABSTRACT: From the reactions between M2(TiPB)4 and two equivalents of the acids LH, the compounds trans-M2(TiPB)2L2 were isolated where M = Mo, compound I, and M = W, II, TiPB = 2,4,6-triisopropylbenzoate, and LH = 4-[2-(4-Pyridinyl)ethenyl]benzoic acid. In related reactions when tris(pentafluorophenyl)boron, A, was added, the Lewis acid− Lewis base complexes Mo 2 (T i PB) 2 (LA) 2 , IB, and W2(TiPB)2(LA)2, IIB, were isolated. Compounds I and IB are red and purple, respectively, while II and IIB are green. The new compounds have been characterized by 1H NMR, UV−visibleNIR absorption spectroscopy, and electrochemical studies, which are tied together with density functional theory, DFT, and timedependent DFT calculations. Chemical reduction of IB and IIB yields anions where the single electron occupies a ligand-based orbital as indicated by EPR spectroscopy. The LUMO and LUMO+1 are ligand-based, and are close in energy, and upon reduction, no IVCT is observed.

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INTRODUCTION When two MM quadruple bonds are linked by a conjugated π system, such as dicarboxylates, the two M2 δ orbitals are split by their interaction with the π bridge. Oxidation of these metallic complexes leads to a situation where one electron is removed from the highest occupied δ orbital and this leads to a mixed valence complex.1 If the electronic coupling is strong this leads to Class III on the Robin and Day Scheme2 where the electron is delocalized over four M centers. In this instance EPR and NIR studies will reveal the case.3 Similarly, if two trans-ligands are conjugated to the M2δ orbital this may lead, upon reduction, to a mixed valence ligand state. Again, if carboxylates are employed this may lead to two orbitals, most commonly the LUMO and LUMO+1, that are ligand-based and the odd electron lies in the LUMO. Again, if the two ligands are strongly coupled via the M2δ orbital this may lead to a mixed valence Class III compound.4 Schematically these can be represented by an orbital energy level diagram as shown in Figure 1. Although many examples of this type of behavior have been seen by Cotton5,6 and by us7 involving mixed valence cations, only one such anionic example has been reported, namely, M2(TiPB)2(O2CC6H4NB(C6F5)3)2.8 Here, upon reduction, the nicotinate based anions were mixed valence of Class III behavior and they showed NIR spectra with sharp absorptions at ∼4000 cm−1. Given the size of the isonicotinate ligands, we assumed that extending the π-complex further would lead to an © XXXX American Chemical Society

Figure 1. Orbital energy diagram showing the interaction between the M2 δ and ligand π*orbitals.

enhancement of the Lπ−M2δ−Lπ conjugation. We describe here our results.

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RESULTS AND DISCUSSION Syntheses. The ligand, 4-[2-(4-Pyridinyl)ethenyl]benzoic acid, LH, was synthesized according to the reaction shown in Scheme 1. 4-Methylpyridine, 4-formylbenzoic acid, and acetic anhydride were refluxed in a Knoevenagel condensation reaction. The 1H NMR spectrum for LH recorded in DMSOd6 is given in the Supporting Information, Figure S1.

Received: July 8, 2015

A

DOI: 10.1021/acs.inorgchem.5b01520 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of LH

Figure 2. Structures of trans-M2(TiPB)2(L)2 (I, M = Mo; II, M = W; top) and trans-M2(TiPB)2(LA)2 (IB, M = Mo; IIB, M = W; bottom). R = 2,4,6-triisopropylphenyl.

Figure 3. Frontier molecular orbitals for I′ (red) and IB′ (purple), with Gaussview contour plots for IB′.

Then, the reaction between M2(TiPB)4, where M = Mo or W and TiPB= 2,4,6-triisopropylbenzoate, and two equivalents of LH gave compounds I, Mo2(TiPB)2L2 and II, W2(TiPB)2L2 in toluene at room temperature within 3 days. The products I and II are red and green, respectively, and are precipitated from solution. They were collected by filtration and were washed with hexanes to remove any residual TiPBH.

In a similar manner, M2(TiPB)4 was reacted with two equivalents of LH and two equivalents of B(C6F5)3, A, to give compounds IB, Mo2(TiPB)2(LA)2, and IIB, W2(TiPB)2(LA)2. These compounds were similarly purified. Compound IB is purple and IIB is dark green. The structures of I, IB, II, and IIB are shown in Figure 2. The new compounds were characterized by 1H and 19F NMR, UV−visible-NIR absorption spectroscopies, MALDIB

DOI: 10.1021/acs.inorgchem.5b01520 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Frontier molecular orbitals for II′ (blue) and IIB′ (green), with Gaussview contour plots for IIB′.11

Figure 5. Gaussview contour plots of molecular orbitals of IB′ containing Lπ* character11

of IB′ and IIB′ requires more energy as the introduction of the Lewis acids that coordinate to the N atoms stabilize the bonding and antibonding orbitals. Also, the HOMO to LUMO gap decreases with the introduction of the B(C6F5)3 groups. These changes were observed previously for M2(O2CH)2(O2CC6H4NB(C6F5)3)2. However, what is remarkably different is the energy between the LUMO and LUMO+1. In the compounds IB′ and IIB′ this is only 0.09 and 0.17 eV for Mo2 and W2, respectively. By contrast, this energy separation for the isonicotinate−B(C6F5)3 adducts was 0.3 eV for Mo2 and 0.4 eV for W2.8 Furthermore, the energy separation between the LUMO and LUMO+1 decreases upon complexation with B(C6F5)3. This is contrary to what we had expected given the much longer extent of conjugation in these molecules. See Figure 1. Looking above the energies of the LUMO and LUMO+1 we find that for Mo2 the LUMO+2 is the M2δ*, but for W2 we see the M2δ* is the LUMO+3. Indeed, for Mo2 we see that the LUMO+3 and LUMO+4 are ligand-based π* orbitals and the LUMO+4 has some M2δ character. Although it is not accurate to quote the energies of the vacant orbital, we do note that the energy difference is notably larger than what we see for the LUMO and LUMO+1. It is probably this mixing that reduces the energy separation of the LUMO and LUMO+1. The

TOF mass spectrometry, and electrochemical CV and DPV experiments. Data are given in the SI Experimental section, and MALDI-TOF-MS plots and 19F NMR spectra are shown in Figures S2 and S3, respectively. Complexes IB and IIB show molecular ion peaks matching I and II, respectively, with the loss of the B(C6F5)3 groups during the MALDI-TOF experiment. The new compounds are proposed to have the trans-geometry as do many related complexes of the type transM2(TiPB)2L2. The ligands TiPB have their aryl groups twisted by ∼90° from their CO2 planes and the Lπ−M2δ−Lπ geometry allows extensive conjugation.9,10 Electronic Structure Calculations. The compounds I′, IB′, II′, and IIB′ were employed where formate was substituted for the TiPB ligands and the full ligand sets, L and LA, were included. The geometry was set to C1 and calculations were computed using density functional theory, DFT, and timedependent DFT. The frontier orbitals for I′ and IB′ are shown in Figure 3 and those for II′ and IIB′ are given in Figure 4. For each molecule the HOMO is the M2δ orbital with a small degree of ligand mixing. The LUMO and LUMO+1 are the in-phase Lπ* orbitals, which have no M2δ character, and the out-of-phase Lπ* character with a small contribution from the M2δ orbital. In going from I′ to IB′ and from II′ to IIB′ we see a stabilization of the frontier orbitals. This means that oxidation C

DOI: 10.1021/acs.inorgchem.5b01520 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry difference between the orbitals having Lπ* character for IB′ is shown in Figure 5. A summary of the calculations of these model compounds is given in Table S1. The time-dependent DFT spectra are reproduced in Figure S4. Agreement with experiment is good for Mo2 complexes but the observed lower energy absorptions for the W2 complexes are based on the approximation of theory for the 5d elements. Electronic Absorption Spectra. The compounds were examined in THF solutions at room temperature, and collectively they are shown in Figure 6. The spectra are

Figure 7. Differential pulse voltammogram of I and II in THF, referenced to Cp2Fe+/0.

Figure 6. 1MLCT electronic absorptions for I (red), IB (purple), II (blue), and IIB (green) in THF.

normalized, whereas in reality the 1MLCT transitions increase with the HOMO−LUMO gap decreasing. The compounds are red (I), purple (IB), and green compounds (II and IIB) as indicated in Figure 6. Also, it is noted that the compounds with the B(C6F5)3 are notably shifted to the red, and compound IIB has some absorption at energies below 1000 nm. Electrochemical Studies. The compounds undergo a reversible oxidation wave corresponding to the removal of an electron from the M2δ orbital. This occurs more easily from compound I and II relative to their B(C6F5)3 compounds. On going from I and IB to their tungsten relatives, the oxidation becomes easier as the HOMO is raised in energy. This is in agreement with and is expected from what has been seen in multiple instances in prior work done for Mo and W quadruply bonded complexes. To more negative potential all four compounds also undergo reductions, which are ligand-based, as indicated by the calculations. Cyclic voltammograms can be found in Figure S5. In complex I we see two one-electron reductions centered around −2.25 and −2.50 V. Upon addition of boron in IB we see a two-electron reduction process which is shifted more positive than the first reduction seen in I. In addition, an additional irreversible one-electron reduction more negative than the two-electron reduction is seen. The two-electron reduction wave is centered at −1.59 V vs Fc/Fc+, as determined by differential pulse voltammetry. The reversible oxidation seen is at a comparable potential to I, centered at 0.03 V, as shown in Figures 7 and 8. In complex II we see reduction features similar to that seen in I, along with the shifted oxidation wave seen at −0.59 V, as is expected on moving from Mo to W. On going from II to IIB

Figure 8. Differential pulse voltammogram comparing IB and IIB in THF, referenced to Cp2Fe+/0.

we see equivalent reduction events as seen in IB. The twoelectron first reduction wave is centered at −1.61 V, while the oxidation potential is seen at −0.50 V (Figures 7 and 8). As can be seen in Figure 8, the reduction of the ligand is a two electron reduction. The size of the current response is twice as large as that of the oxidation which removes just one electron from the δ orbital. This is quite contrary to what was seen before in the B(C6F5)3 adducts of the isonicotinate ligands bound to M2 centers. Indeed, this reduction implies that there is very little coupling between the ligands. Also, one other feature is that while the DFT calculations indicate that the addition of B(C6F5)3 stabilizes the δ orbital by ∼0.5 V, we see very little change in the oxidations of I and IB and of II and IIB. Again, this is quite different from the isonicotinates bound to M2 centers. A most plausible explanation for the latter has to do with the relative degree of solvation of the two systems. With the notable larger ligand effects in the new compounds solvation effects may stabilize the oxidation potentials. We have not pursued this interaction by computational methods, in part because of the size of the molecules. Synthesis of Radical Anions. The reduction of complexes IB and IIB was accomplished by the addition of Co(Cp*)2 to the solutions of the above in THF. In both instances there was a color change. For IB to IB− there was a change from purple D

DOI: 10.1021/acs.inorgchem.5b01520 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry to orange−red, while for IIB to IIB− the color changed from green to blue−purple. The changes in color are shown in Figure 9 for IB− and Figure 10 for IIB−. To determine that

with a broad shoulder residing in the green. The use of 1.0 equiv stoichiometrically showed similar results for the appearance of the new band in the 600 nm area and a depletion of the shoulder from the neutral complex at 860 nm. The reduced species were also examined by EPR spectroscopy, and as shown in Figure 11 there were single sharp resonances close to 3300 G. The lack of any coupling to 95/97 Mo, I = 5/2, and 183W, I = 1/2, makes these signals characteristic of an organic radical anion, with a g value of ∼2.00. The reduced species did not show any features in the NIR. No bands similar to those seen for the compounds M2(TiPB)2[(O2CC6H4NB(C6F5)3)2]− were seen, which had bands close to 4000 cm−1. Concluding Remarks. Mixed valency is an extended theory that involves inorganic and organic molecules, and in mixed ligand−metal systems it can also include mixed valence on ligands. In organic chemistry the mixed valence species include anions derived from organic conjugated systems with πacceptors such as NO2 groups12 and cations where πconjugated groups separate redox active amino groups.13 Ligands attached to metals that can show mixed valency are rather rare, but they include the molecular squares of Re atoms separated by bridges such as bipyridyl groups, and as metals that contain partially reduced ligands such as quinones.14−16 In our case, the use of the M2 δ orbital to link two conjugated πsystems, as in the case of the isonicotinate ligands, can lead to a situation where the seemingly simple orbital picture depicted in Figure 1 shows how the LUMO and LUMO+1 are split by the interaction with the M2 δ orbital. In our case, by extending the π-system further as in Scheme 1, the LUMO and LUMO+1 are not split by a significant amount of energy. Indeed, placing the B(C6F5)3 units decreases this energy separation. As we show here, the reduced complexes contain an electron in an organic π-system but there is no evidence of an IVCT transition either at low energy, a charge resonance band typical of a Class III complex, or at higher energy with a broad transition typical of a Class II compound. The problem seems to arise because there are now more ligand π systems involved. Indeed, as we show from the DFT calculations, there are higher energy orbitals that are in- and out-of-phase that mix with the M2 δ orbital. Thus, the simple criteria that we have used for the coupling of two M2 centers by a small bridge (e.g., oxalate, terephthalate) and for two small ligands (e.g., isonicotinate ligands), coupled to one

Figure 9. Shifting of 1MLCT to 2MLCT band for IB− in THF.

Figure 10. Shifting of 1MLCT to 2MLCT band for IIB− in THF.

these transitions in the visible are truly a one electron reduction, the stoichiometric amount of Co(Cp*)2 was varied. Using slightly less than 1.0 equiv of Co(Cp*)2 with 1.0 equiv of IIB caused a shift from the band centered at 860 to 600 nm,

Figure 11. EPR spectra of IB− (left) and IIB− (right) in THF/hexanes at 298 K. E

DOI: 10.1021/acs.inorgchem.5b01520 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry M2 δ orbital does not produce a splitting of the LUMO and LUMO+1. Does this mean that the two ligands are not strongly coupled? No, we do not think so. The low-energy MLCT transition and the more extensive M2 δ and ligand π* systems simply means that a Class III mixed valence system cannot be seen by the LUMO to LUMO+1 electronic transition. Perhaps a more extensive mixing of the ligands can be seen by timeresolved infrared spectroscopy, TRIR. Photoexcitation will remove an electron from the M2 δ orbital and place it in the LUMO. Thus, in the case of a ligand such as that shown above

(6) Cotton, F. A.; Murillo, C. A.; Villagrán, D.; Yu, R. J. Am. Chem. Soc. 2006, 128 (10), 3281−3290. (7) Chisholm, M. H. Philos. Trans. R. Soc., A 2008, 366 (1862), 101− 112. (8) Bunting, P.; Chisholm, M. H.; Gallucci, J. C.; Lear, B. J. J. Am. Chem. Soc. 2011, 133 (15), 5873−5881. (9) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Spilker, T. F.; Young, P. J. Dalton Trans. 2013, 42 (40), 14491−14497. (10) Brown-Xu, S. E.; Chisholm, M. H.; Durr, C. B.; Spilker, T. F.; Young, P. J. Chem. Sci. 2015, 6 (3), 1780−1791. (11) Dennington, R.; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W. L.; Gilliland, R. GaussView; Semichem Inc.: Shawnee Mission, KS, 2003. (12) Nelsen, S. F.; Weaver, M. N.; Telo, J. P. J. Am. Chem. Soc. 2007, 129 (22), 7036−7043. (13) Lambert, C.; Nöll, G. J. Am. Chem. Soc. 1999, 121 (37), 8434− 8442. (14) Dinolfo, P. H.; Coropceanu, V.; Brédas, J.-L.; Hupp, J. T. J. Am. Chem. Soc. 2006, 128 (39), 12592−12593. (15) Dinolfo, P. H.; Hupp, J. T. J. Am. Chem. Soc. 2004, 126 (51), 16814−16819. (16) Dinolfo, P. H.; Williams, M. E.; Stern, C. L.; Hupp, J. T. J. Am. Chem. Soc. 2004, 126 (40), 12989−13001. (17) Alberding, B. G.; Chisholm, M. H.; Gallucci, J. C.; Ghosh, Y.; Gustafson, T. L. Proc. Natl. Acad. Sci. U. S. A. 2011, 108 (20), 8152− 8156.

Figure 12. Ligand used to investigate the mixed valency of these systems by use of TRIR.

(Figure 12), we may examine the M2 δ to ligand π*. The 1 MLCT or S1 state has υ (CC) changes. If the coupling is strong we will see just one υ (CC) vibration at a position calculated for the anion. If the charge is localized on just one ligand we will see either two υ (CC) or one just twice as low as if it were localized.17 Current investigation is underway. Finally, this work shows that the lack of an intervalence band, or a charge resonance band, according to Hush’s rule, cannot be seen even if two metals were separated by M2(O2C)2 units. The two metals could be strongly linked but the separation of the LUMO and LUMO+1 would not indicate this.

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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.5b01520. Experimental section, materials and methods, supporting figures, and table (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. Funding

We thank the National Science Foundation, grants CHE1266298 and CHE-0957191, for financial support, and the Ohio Supercomputing Center for computational resources provided. Also, we thank the CCIC Mass Spectrometry and Proteomics Lab for use of their MALDI instrumentation, supported by NIH Awards One S10 RR025660-01A1 and P30 CA016058. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b01520 Inorg. Chem. XXXX, XXX, XXX−XXX