J. Phys. Chem. A 2011, 115, 39–46
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Matrix Isolation Spectroscopic and Theoretical Study of Dihydrogen Activation by Group V Metal Dioxide Molecules Mingfei Zhou,*,† Caixia Wang,‡ Jia Zhuang,† Yanying Zhao,‡ and Xuming Zheng‡ Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, China, and Department of Chemistry, Zhejiang Sci-Tech UniVersity, Hanzhou, China ReceiVed: October 4, 2010; ReVised Manuscript ReceiVed: NoVember 26, 2010
The reactions of group V metal dioxide molecules with dihydrogen have been studied by matrix isolation infrared spectroscopy. The ground state VO2 molecule is able to cleave dihydrogen heterolytically and spontaneously in forming the HVO(OH) molecule in solid argon. In contrast, the reaction of VO2 with dideuterium to form DVO(OD) proceeds only under UV-visible excitation via a weakly bound VO2(η2-D2) complex. Theoretical calculations predict that the dihydrogen cleavage process is thermodynamically exothermic with a small barrier. The niobium and tantalum dioxide molecules react with dihydrogen to give primarily the side-on bonded metal dioxide bis-dihydrogen complexes, NbO2(η2-H2)2 and TaO2(η2-H2)2, which are further transferred to the HNbO(OH) and HTaO(OH) molecules via photoisomerization in combination with H2 elimination under UV-visible light excitation. Introduction Catalytic hydrogenation is one of the most important reactions in industry.1 The activation of dihydrogen by metal centers is a fundamental step in nearly all metal catalytic hydrogenation reactions.2 Dihydrogen activation at metal centers has received considerable attention.3-8 Transition metal oxides are vital heterogeneous catalysts and/or supports in many processes involving H2. The reactions of transition metal oxide molecules with dihydrogen may serve as simple models in understanding the mechanisms of dihydrogen activation by transition metals in some catalytic and biochemical processes. The reactions of bare transition metal oxide cations with dihydrogen have been studied both experimentally and theoretically.9-19 Mass spectrometric studies in the gas phase indicate that thermalized MnO+ reacts very efficiently with H2 to eliminate either a H radical or H2O via H atom abstraction mechanism.14 However, the reactions of FeO+ and CoO+ cations with dihydrogen are quite inefficient considering the favorable thermochemistry for these reactions.13,15 Although thermalized CrO+ does not react with H2,16 the high-valent chromium dioxide cation slowly reacts with H2 to form CrO+ and H2O as products.17 Theoretical calculations of the H2 activation by FeO+ indicate that the addition-elimination mechanism involving 2 + 2 addition in the bond activation step and the rebound mechanism involving the H-abstraction followed by a barrierless rebound of the H radical are competitive and both exhibit two-state reactivity with a crossing between the high-spin and low-spin surfaces along the reaction coordinate.18 In contrast to the cation reaction systems, the neutral transition metal oxide reactions have gained much less attention.20-25 Gas phase kinetic study shows that the ground state FeO molecule is unreactive toward H2.20 Matrix isolation infrared spectroscopic study suggests that transition metal oxide molecules such as MnO, FeO, and CrO2 are able to cleave dihydrogen heterolyti* To whom correspondence should be addressed. E-mail: mfzhou@ fudan.edu.cn. † Fudan University. ‡ Zhejiang Sci-Tech University.
cally under UV-visible excitation.21-23 Recently, the reactions of tantalum oxides in different oxidation states (TaIIO, TaIVO2, and TaVO4) with dihydrogen were studied in this laboratory. The results indicate that the neutral TaO4 d0 complex is able to cleave dihydrogen heterolytically and spontaneously in forming the HTaO(OH)(η2-O2) complex in solid argon at cryogenic temperatures.24 In this paper, we report a combined matrix isolation infrared spectroscopic and theoretical study of the reaction of group V metal dioxide molecules with dihydrogen. Experimental and Computational Methods In this report, the reactions of group V metal dioxides with dihydrogen are studied by matrix isolation infrared absorption spectroscopy, and metal dioxide molecules were prepared by pulsed laser evaporation of bulk metal oxide targets. The experimental setup for pulsed laser-evaporation and matrix isolation infrared spectroscopic investigation has been described in detail previously.26 Briefly, the 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz repetition rate) was focused onto a rotating metal oxide target through a hole in a CsI window cooled normally to 4 K by means of a closedcycle helium refrigerator. The laser-evaporated metal oxide species were codeposited with dihydrogen in excess argon onto the CsI window. In general, matrix samples were deposited for 1 h at a rate of approximately 5 mmol/h. The V2O5 and Nb2O5 bulk targets were prepared from sintered metal oxide powder. The H2/Ar mixtures were prepared in a stainless steel vacuum line using a standard manometric technique. Isotopic-labeled D2 (Cambridge Isotope Laboratories, 99.8%) and HD (Isotec, 98%) samples were used without further purification. The infrared absorption spectra of the resulting sample were recorded on a Bruker Vertex 80 V spectrometer at 0.5 cm-1 resolution between 4000 and 450 cm-1 using a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. After the infrared spectrum of the initial deposition had been recorded, the samples were warmed up to the desired temperature and quickly recooled and more spectra were taken. Selected samples were also subjected to broad-band irradiation using a high-pressure mercury arc lamp with glass filters.
10.1021/jp109498b 2011 American Chemical Society Published on Web 12/13/2010
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Figure 1. Infrared spectra in the 1200-700 cm-1 region from codeposition of laser-evaporated vanadium oxides with 2.0% H2 in argon: (a) 1 h of sample deposition at 4 K, (b) after 25 K annealing, (c) after 30 K annealing, and (d) after 20 min of visible light irradiation (λ > 500 nm).
Quantum chemical calculations were performed to determine the molecular structures and to help the assignment of vibrational frequencies of the observed reaction products. The calculations were performed at the level of density functional theory (DFT) with the B3LYP method, where Becke’s three-parameter hybrid functional and the Lee-Yang-Parr correlation functional were used.27 The 6-311++G** basis set was used for the H, O, and V atoms, and the SDD pseudopotential and basis set was used for Nb.28,29 The geometries were fully optimized, and the harmonic vibrational frequencies were calculated with analytic second derivatives. The zero-point energies (ZPE) were derived. Transition state optimizations were done with the synchronous transit-guided quasi-Newton (STQN) method and were verified through intrinsic reaction coordinate (IRC) calculations. All these calculations were performed by using the Gaussian 09 program.30 Results and Discussion Infrared Spectra. The vanadium dioxide molecules were prepared by pulsed laser evaporation of bulk V2O5 target. Pulsed laser evaporation of bulk V2O5 target under controlled laser energy followed by condensation with pure argon formed the VO (983.6 cm-1) and VO2 (ν3, 936.0 cm-1 and ν1, 946.3 cm-1) molecules as major products.31 A group of absorptions at 1121.9, 975.3, 974.1, and 555.6 cm-1 were produced on annealing, which were attributed to the VO4 complex.31 Both the VO2 and VO4 molecules were characterized to be coordinated by argon atom(s) in solid argon matrix.32 Figures 1 and 2 show the spectra in selected regions from codeposition of laser-evaporated vanadium oxides with a H2/Ar sample (2.0% H2 molar ratio). Besides the VO, VO2 and VO4 absorptions, two groups of new absorptions were produced (labeled as A and B in Figures 1 and 2). The group A absorptions markedly increased at the expense of the VO2 absorptions when the sample was annealed to 25 K. The group B absorptions were produced at the expense of group A absorptions on 30 K annealing. The group B absorptions decreased, whereas the group A absorptions increased when the sample was subjected to visible light irradiation (λ > 500 nm). The experiment was repeated using a D2/Ar sample (2.0% D2) as reagent gas, and the resulting spectra in selected region are shown in Figure 3. In this experiment, a
Zhou et al.
Figure 2. Infrared spectra in the 3800-3600 and 1800-1680 cm-1 regions from codeposition of laser-evaporated vanadium oxides with 2.0% H2 in argon: (a) 1 h of sample deposition at 4 K, (b) after 25 K annealing, (c) after 30 K annealing, and (d) after 20 min of visible light irradiation (λ > 500 nm).
Figure 3. Infrared spectra in the 1280-900 cm-1 region from codeposition of laser-evaporated vanadium oxides with 2.0% D2 in argon: (a) 1 h of sample deposition at 4 K, (b) after 30 K annealing, and (c) after 20 min of UV-visible light irradiation (250 < λ < 580 nm).
group of new absorptions (labeled as C) appeared on annealing and decreased upon UV light irradiation (250 < λ < 580 nm), during which the group A absorptions were formed. The band positions of the newly observed product absorptions are listed in Tables 1-3. Pulsed laser evaporation of bulk Nb2O5 target under controlled laser energy followed by condensation with pure argon formed only the NbO (970.6 cm-1) and NbO2 (ν3, 875.9/869.8 cm-1 and ν1, 933.5/931.1 cm-1) molecules.33 Experiments were performed using the H2/Ar sample as reagent gas. The spectra in selected regions from codeposition of laser evaporated niobium oxides with 2.0% H2 in argon are shown in Figures 4 and 5, respectively. After 1 h of sample deposition at 4 K, strong NbO and NbO2 absorptions were observed. In addition, two groups of new product absorptions were produced upon sample annealing and photolysis (labeled as D and E in Figures 4 and 5). The absorptions of group D appeared on sample annealing, but disappeared on broad-band UV-visible irradiation (300 < λ < 580 nm). The group E absorptions were produced upon broad-band irradiation (300 < λ < 580 nm). Experiments using
Dihydrogen Activation by Group V Metal Dioxides
J. Phys. Chem. A, Vol. 115, No. 1, 2011 41
TABLE 1: Observed Argon Matrix and Calculated Vibrational Frequencies (cm-1) for HVO(OH) (A) HVO(OH)
DVO(OH)
a
mode
obs
O-H str V-H str VdO str V-OH str V-H bend VOH bend HVOH bend
3708.0/3703.7 1727.5/1724.9 1020.1/1019.3 745.3/740.6
calcd
540.5
b
3904.4 (208) 1811.9 (242) 1090.7 (238) 764.9 (148) 624.7 (14) 508.7 (163) 406.8 (154)
a Two matrix trapping sites were observed. listed.
b
DVO(OD)
obs
calcd
obs
calcd
3708.1/3703.9 1248.8/1246.9 1019.6/1018.8 736.5/730.2
3904.4 (209) 1295.9 (132) 1089.4 (230) 755.5 (140) 462.1 (15) 506.9 (160) 393.9 (141)
2735.4/2731.9 1248.8/1246.8 1019.5/1018.7 733.9/729.6
2844.6 (138) 1295.9 (131) 1089.4 (230) 739.1 (161) 460.2 (8) 402.1 (91) 327.5 (82)
540.4
The intensities are listed in parentheses in km/mol. Only the vibrations above 400 cm-1 are
TABLE 2: Observed Argon Matrix and Calculated Vibrational Frequencies (cm-1) for HVO(OH)(η2-O2) (B) HVO(OH)(η2-O2)
DVO(OH)(η2-O2)
mode
obs
calcda
obs
calcd
O-H str V-H str O-O str VdO str V-OH str V-H bend V-H bend V-O2 str + VOH bend V-O2 str + V-OH bend V-OH wagging
3644.4 1756.4 1142.6 1041.7 816.2 731.4
3644.4 1275.5 1142.4 1041.7 804.6
557.8
3864.8 (234) 1907.3 (86) 1226.8 (61) 1118.7 (187) 848.2 (60) 768.5 (73) 675.1 (12) 562.8 (90)
3864.8 (234) 1365.4 (47) 1225.8 (61) 1117.7 (187) 805.2 (128) 592.4 (34) 507.3 (46) 558.0 (59)
509.7
534.9 (108)
534.5 (106)
483.5 (65)
475.4 (34)
552.4
a
The intensities are listed in parentheses in km/mol. Only the vibrations above 400 cm-1 are listed.
TABLE 3: Observed and Calculated Vibrational Frequencies (cm-1) for VO2(η2-H2) (C) VO2(η2-H2) mode H-H str V-H2 asym str OVO sym str OVO asym str V-H2 sym str VH2 torsion
obs
calcd
VO2(η2-D2) a
3830.8 (151) 1135.9 (6) 1033.3 (53) 1025.1 (455) 729.8 (52) 459.3 (0)
obs
calcd
947.0 940.9
2710.0 (75) 804.7 (2) 1030.0 (63) 1025.0 (456) 528.1 (21) 325.7 (0)
Figure 4. Infrared spectra in 1160-660 cm-1 region from codeposition of laser-evaporated niobium oxides with 2% H2 in argon: (a) 1 h of sample deposition at 4 K, (b) after 25 K annealing, and (c) after 20 min of UV-visible irradiation (300 < λ < 580 nm).
a The intensities are listed in parentheses in km/mol. Only the vibrations above 400 cm-1 are listed. The vibrational frequencies of the 2A1 ground state of VO2 were predicted at 1026.5 (ν1), 992.3 (ν3), and 261.5 (ν2) cm-1, and were observed at 946.4 (ν1) and 936.0 (ν3) cm-1 in solid argon.
the D2/Ar, HD/Ar and H2 + D2/Ar samples were also done. The difference spectra in selected regions are shown in Figures 6 and 7, respectively. The band positions of the newly observed product absorptions are listed in Tables 4 and 5. The spectra from the reaction of tantalum dioxide and dihydrogen are very similar to those of the NbO2 + H2 reaction. The results on the TaO2 + H2 reaction have recently been reported24 and will not repeated here. HVO(OH) (A). Five vibrational modes were observed for absorber A, and each mode involves two absorptions due to site splitting. The 3708.0/3703.7 cm-1 absorptions shifted to 2735.4/2731.9 cm-1 with D2. The band position and H/D isotopic frequency ratio (1.356) indicate that these absorptions are due to O-H stretching vibration. The 1727.5/1724.9 absorptions shifted to 1248.8/1246.8 cm-1 in the experiment with D2, which defines a H/D isotopic frequency ratio of 1.383. The band position and deuterium isotopic shift indicate that the
Figure 5. Infrared spectra in 3780-3660 and 1780-1640 cm-1 regions from codeposition of laser-evaporated niobium oxides with 2% H2 in argon: (a) 1 h of sample deposition at 4 K, (b) after 25 K annealing, and (c) after 20 min of UV-visible irradiation (300 < λ < 580 nm).
1727.5/1724.9 cm-1 absorptions are originated from a V-H stretching vibration. The 1020.1/1019.3 cm-1 absorptions showed very small shifts with D2, suggesting that they are due to terminal VdO stretching mode. The band position is only slightly lower than the VdO stretching mode of the H2VO molecule observed at 1029.4 cm-1 in solid argon.34 These
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Zhou et al. TABLE 4: Observed Argon Matrix and Calculated Vibrational Frequencies (cm-1) for NbO2(η2-H2)2 (D) NbO2(η2-H2)2 mode
Figure 6. Difference IR spectra in 3820-2200 cm-1 region from codeposition of laser-evaporated niobium oxides with isotopic-labeled samples in excess argon [spectrum taken after 20 min of UV-visible irradiation (300 < λ < 580 nm) minus spectrum taken after 25 K annealing]: (a) 2.0% H2, (b) 2.0% D2, (c) 1.0% H2 + 1.0% D2, and (d) 2.0% HD.
H-H sym str (a1) H-H asym str (b2) HNbH sym str (a1) HNbH asym str (b2) NbO2 sym str (a1) NbO2 asym str (b1) Nb-H2 sym str (a1) Nb-H2 asym str (b2) Nb-H2 sym bend (a2) Nb-H2 asym bend (b1)
obsa
NbO2(η2-D2)2
calcdb
obs
calcd
3545.5 (231)
2363.3
2508.1 (116)
3110.5
3470.2 (553)
2337.1
2455.0 (277)
1141.1/1138.0
1250.1 (12)
836.2/834.0
884.6 (5)
1135.1/1131.5
1228.4 (46)
831.2/828.7
869.3 (23)
927.2
943.1 (57)
927.3
942.5 (64)
869.1
902.3 (285)
865.3
898.5 (285)
712.9/708.0
820.8 (39)
536.8/533.1
587.8 (16)
683.9/682.5
782.6 (97)
515.4/514.2
561.8 (44)
661.6 (0)
468.4 (0)
598.2 (0)
425.2 (0)
a Two matrix trapping sites were observed for most modes. b The intensities are listed in parentheses in km/mol. Only the vibrations above 400 cm-1 are listed.
TABLE 5: Observed Argon Matrix and Calculated Vibrational Frequencies (cm-1) for HNbO(OH) (E) HNbO(OH) a
mode
obs
O-H str Nb-H str NbdO str Nb-OH str Nb-H bend NbOH bend HNbOH bend
3714.6/3708.9 1702.0/1695.3 966.3/962.9 696.9/690.0 473.3
calcd
DNbO(OD) b
3907.5 (227) 1791.4 (288) 985.2 (185) 712.2 (129) 596.5 (19) 525.7 (151) 405.5 (126)
obs
calcd
2740.2/2735.6 1223.7/1219.0 966.3/962.9 671.0/662.6
2846.8 (149) 1274.9 (149) 983.6 (177) 680.3 (152) 440.0 (7) 410.7 (75) 319.5 (69)
a Two matrix trapping sites were observed for most modes. b The intensities are listed in parentheses in km/mol. Only the vibrations above 400 cm-1 are listed.
Figure 7. Difference IR spectra in the 1750-1600 and 1250-800 cm-1 regions from codeposition of laser-evaporated niobium oxides with isotopic-labeled samples in excess argon [spectrum taken after 20 min of UV-visible irradiation (300 < λ < 580 nm) minus spectrum taken after 25 K annealing]: (a) 2.0% H2, (b) 2.0% D2, (c) 1.0% H2 + 1.0% D2, and (d) 2.0% HD.
spectral features lead us to propose the assignment of group A absorptions to HVO(OH) (Table 1). The 745.3/740.6 cm-1 absorptions are due to the V-OH stretching mode. Density functional theory calculations were performed to support the experimental assignment. The HV(O)OH molecule was predicted to have a doublet ground state with a nonplanar C1 structure (Figure 8). The O-H, V-H, VdO, and V-OH stretching modes were computed at 3904.4, 1811.9, 1090.7, and 764.9 cm-1 with deuterium isotopic shifts in good agreement with the experimental values (Table 1). HVO(OH)(η2-O2) (B). The group B absorptions are assigned to different vibrational modes of the HVO(OH)(η2-O2) complex (Table 2). The 3644.4, 1756.4, and 1041.7 cm-1 absorptions are due to the O-H, V-H, and VdO stretching vibrations, which are slightly shifted from those of the HVO(OH) molecule. The 1142.6 cm-1 absorption is due to the O-O stretching vibration. The same mode of VO4 was observed at 1121.9 cm-1
Figure 8. Optimized structures (bond lengths in angstroms, bond angles in degrees) of the species involved in the VO2 + H2 reaction.
in solid argon. The experimental identification of HVO(OH)(η2O2) is further supported by density functional theory calculations. As shown in Figure 8, the HVO(OH)(η2-O2) molecule was calculated to have a doublet ground state without symmetry. The O2 subunit was predicted to be side-on bonded to the vanadium metal center with two slightly inequivalent V-O
Dihydrogen Activation by Group V Metal Dioxides bonds. The O-O bond length was predicted to be 1.307 Å. The observed O-O stretching vibrational frequency as well as the predicted O-O bond length indicate that HVO(OH)(η2-O2) is due to a superoxide complex.35-37 Thus, the complex can be formally described as [HVO(OH)]+[O2]-, with vanadium in its highest +V oxidation state. The calculated vibrational frequencies and intensities for HVO(OH)(η2-O2) as well as isotopicsubstituted DVO(OH)(η2-O2) are listed in Table 2, which provide strong support for the proposed identification of this complex. The VdO, V-H, and V-OH stretching vibrations of the above characterized HVO(OH) with vanadium in +IV oxidation state are slightly lower than the corresponding modes of the HVO(OH)(η2-O2) complex, whereas the O-H stretching mode of HVO(OH) is higher than that of HVO(OH)(η2-O2). These observations imply that the VdO, V-OH, and V-H bonds in HVO(OH)(η2-O2) are stronger than those in HVO(OH) due to increased electrostatic interactions. VO2(η2-D2) (C). The 947.0 and 940.9 cm-1 absorptions (group C) were observed only in the experiments with the D2/ Ar sample. These absorptions are 0.7 and 4.9 cm-1 blue-shifted from the symmetric and antisymmetric OVO stretching vibrations of the vanadium dioxide molecule in solid argon, which suggests that the 947.0 and 940.9 cm-1 absorptions are due to a complex of VO2. Note that the above assigned DVO(OD) absorptions were produced only under UV light irradiation at the expense of group C absorptions. This indicates that the absorber of group C absorptions is the precursor for the formation of DVO(OD), suggesting that the absorber is most likely due to a VO2-D2 complex. In the experiments with H2, the HVO(OH) absorptions were formed spontaneously on annealing; thus, the corresponding VO2-H2 absorptions were not observed. The VO2(H2) complex was predicted to have a 2B1 ground state with C2V symmetry, in which the VH2 plane is perpendicular to the VO2 plane (Figure 8). Similar to the previously characterized metal dihydrogen complexes,2,38 the H2 ligand is side-on bonded to the vanadium metal center in the VO2(H2) complex. The H-H bond length was calculated to be 0.778 Å, lengthened only by about 0.034 Å compared with that of free H2 calculated at the same level of theory. The symmetric and antisymmetric OVO stretching vibrations of VO2(H2) were computed at 1033.3 and 1025.1 cm-1 (Table 3). The same modes for VO2(D2) were calculated to be 1030.0 and 1025.0 cm-1, respectively, which are about 3.5 and 32.7 cm-1 higher than those of VO2 calculated at the same level of theory. NbO2(η2-H2)2 (D). The group D absorptions in the niobium oxide experiments are assigned to different vibrational modes of the NbO2(η2-H2)2 complex (Table 4). The 927.2 and 869.1 cm-1 absorptions are due to the symmetric and antisymmetric ONbO stretching modes, which are about 6.3 and 6.8 cm-1 redshifted from the corresponding absorptions of NbO2 in solid argon. The symmetric and antisymmetric H-H stretching modes were observed at 3229.0 and 3110.5 cm-1, while the corresponding D-D stretching modes were observed at 2363.3 and 2337.1 cm-1. Besides the 2363.3 and 2337.1 cm-1 absorptions, an additional intermediate absorption at 2348.6 cm-1 was clearly resolved when the H2 + D2 (1:1 molar ratio) mixture was used (Figure 6, trace c), which confirms the involvement of two equivalent H2 fragments in absorber D. The 1141.1, 1135.1, 712.9, and 683.9 cm-1 absorptions are attributed to the symmetric and antisymmetric HNbH and Nb-H2 stretching vibrations (Table 4).
J. Phys. Chem. A, Vol. 115, No. 1, 2011 43
Figure 9. Optimized structures (bond lengths in angstroms, bond angles in degrees) of the species involved in the NbO2 + H2 reactions.
The NbO2(η2-H2)2 complex was predicted to have a 2A1 ground state with C2V symmetry (Figure 9), which can be viewed as being formed by the interaction of the ground state NbO2 (2A1) and 2H2. Similar to the previously characterized metal dihydrogen complexes,2,38 the H2 subunits are side-on bonded to the niobium metal center and lie in the plane that is perpendicular to the ONbO plane. The H-H bond length was calculated to be 0.798 Å, lengthened only by about 0.054 Å compared with that of free H2. As listed in Table 4, the harmonic frequencies calculated at the B3LYP level of theory deviate from the experimental values as the anharmonicity of the vibrations is not taken into consideration. The vibrations of dihydrogen complexes generally show large anharmonicity.23,39,40 The relative IR intensities of the H-H stretching modes are overestimated, as reported previously.39 No absorptions due to the NbO2(η2-H2) complex were observed in the experiments. Geometry optimization on the NbO2(η2-H2) complex structure almost converged to separated NbO2 + H2, indicating that NbO2 is not able to form a stable 1:1 complex with H2. It is quite interesting to note that the VO2 molecule interacts with dihydrogen to form the 1:1 VO2(η2H2) complex, whereas both the NbO2 and TaO2 molecules must take up two H2 molecules. In general, the bonding interaction between dihydrogen and transition metal centers in transition metal dihydrogen complexes involves both donation of the bonding σ electrons of dihyrogen to vacant metal d orbital and back-donation of metal d electrons to the antibonding orbital of H2. The group V metal dioxide molecules have an 2A1 ground state with an electron configuration of (core)(a1)1(b1)0. The highest singly occupied (HOMO) a1 orbital is primarily a hybrid of the metal valence s and d orbitals that is polarized away from the O atoms (Figure 10 a). The lowest unoccupied (LUMO) b1 orbital is largely metal d orbital and is oriented in the plane perpendicular to the molecular plane (Figure 10 b). Due to σ
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Figure 10. 3D contours of the molecule orbitals of (a) the highest occupied molecular orbital (HOMO) of 2A1 state VO2, (b) the lowest unoccupied molecular orbital (LUMO) of 2A1 state VO2, and (c) the HOMO of 2A1 state NbO2(η2-H2)2.
repulsion, the 2A1 ground state metal dioxide molecules are unbound with dihydrogen. The VO2(η2-H2) complex can be viewed as being formed by the interaction of the 2B1 first excited state VO2 fragment with H2. The 2B1 excited state VO2 molecule has an electron configuration of (core)(b1)1(a1)0, which was predicted to be only 1.1 kcal/mol higher in energy than the 2A1 ground state at the B3LYP/6-311+G* level of theory. Upon 2 A1 to 2B1 promotion, the empty σ symmetry a1 orbital is able to accept σ donation from H2, while the singly occupied b1 orbital serves as an orbital for back-donation from VO2 to H2. Therefore, the ground state VO2(η2-H2) complex was estimated to be bound by 8.4 kcal/mol with respect to the VO2(2B1) + H2(1Σg) asymptote. In contrast to VO2, the 2B1 first excited state of NbO2 and TaO2 was predicted to be 15.2 and 33.4 kcal/mol higher in energy than the 2A1 ground state, too high to favor the 2A1 to 2B1 promotion for bonding. The NbO2(η2-H2)2 and TaO2(η2-H2)2 complexes were predicted to have a 2A1 ground state that correlates to the ground state of metal dioxide molecules. Both complexes are very weakly bound with a binding energy of 4.9 (Nb) and 1.8 (Ta) kcal/mol per H2 molecule, respectively. As has been discussed for the TaO2(η2H2)2 complex,24 the highest singly occupied molecular orbital (HOMO) of NbO2(η2-H2)2 or TaO2(η2-H2)2 is a bonding orbital between the σH-H* orbitals of the two H2 molecules and the d orbital of metal, and the two σH-H* orbitals have significant overlap with each other (Figure 10 c). It is the overlap between the two σH-H* orbitals that enhances the interaction between the H2 molecules and the metal dioxide moiety in the metal dioxide-bisdihydrogen complexes. The analogous VO2(η2-H2)2 complex was predicted to have a 2A1 ground state with strong OVO and H-H stretching vibrations at 1013.4 and 3870.0 cm-1. No absorptions due to VO2(η2-H2)2 were observed in the experiments. HNbO(OH) (E). The group E absorptions involve five vibrational modes with each mode having two absorptions due to site splitting. The spectral features are quite similar to those of HVO(OH). The 3714.6/3708.9, 1702.0/1695.3, 966.3/962.9, and 696.9/690.0 cm-1 absorptions are attributed to the O-H, Nb-H, NbdO, and Nb-OH stretching vibrations, respectively, on the basis of their band positions and deuterium isotopic shifts (Table 5). Note that absorber E was produced under UV-visible light irradiation (300
