ARTICLE pubs.acs.org/JPCA
Tetrahydrometalate Species VH2(H2), NbH4, and TaH4: Matrix Infrared Spectra and Quantum Chemical Calculations Xuefeng Wang† and Lester Andrews*,‡ † ‡
Department of Chemistry, Tongji University, Shanghai, 200092, China Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, United States ABSTRACT: Laser ablated V, Nb, and Ta atoms react with molecular hydrogen in excess neon at 4 K to give vanadium, niobium, and tantalum dihydrides that further react with H2 to form VH2(H2), NbH4, and TaH4. The reaction products are identified by deuterium and deuterium hydride isotopic substitution. DFT and CCSD theoretical calculations are used to predict energies, geometries, and vibrational frequencies for these novel metal hydrides complex and molecules. The vanadium dihydride hydrogen complex, VH2(H2), is identified, while the niobium and tantalum tetrahydrides, NbH4 and TaH4, with D2d symmetry structures are confirmed. Reactions of group 5 metal atoms with H2 condensing in solid hydrogen gave VH2(H2) and the higher tetrahydridehydrogen complexes NbH4(H2)4 and TaH4(H2)4.
’ INTRODUCTION Synthesis of new transition metal hydrides and hydrogen complexes and understanding their structures and bonding are of fundamental importance for new hydrogen-storage materials and for use in the activation of HH, CH, and CC bonds.16 For example terminal tantalum hydrides are extremely reactive, and they can reduce CO2 to form bridging methylene diolate complexes.7 Pulse-laser ablation is one of the most promising methods to generate metal polyhydrides and complexes from reactions of reactive metal atoms with H2.811 Many transition metal hydrides, lanthanide hydrides, and actinide hydrides and their hydrogen complexes have been synthesized for infrared investigation in this research laboratory.913 The IR spectra of the VH2 molecule have been investigated in argon and krypton matrices by Xiao et al.,14 and later ESR spectra suggested that VH2 is linear with a quartet ground state.15 The high-spin VH2H2 complex was calculated to be lower in energy than the low-spin VH4 molecule since the dσ* back-donation was weak for high-spin VH2 that cannot dissociate;16 however, neither VH2H2 nor VH4 has been observed by experiment. The VH3 molecule has also been computed.17 Infrared and electron spin resonance spectra for Nb and Ta hydride molecules have been investigated in solid argon, and antisymmetric and symmetric modes were observed at 1611 and 1569 cm1 for NbH2, but NbH2 was not observed via ESR. The NbH4 molecule proves to have a structure with D2d symmetry through the ESR spectrum, but the assignment of NbH4 from infrared spectra was very tentative.15 The ESR spectra for TaH2 and TaH4 are even more complicated, and analysis of the spectrum is difficult, although the structures are presumed to be similar to those for NbH2 and NbH4.15 We report here the identification of group 5 metal tetrahydrides and hydrogen complexes that are produced through laserablated V, Nb, and Ta atom reactions with molecular hydrogen in solid neon, argon, and hydrogen during condensation at 4 K. r 2011 American Chemical Society
Quantum chemical calculations are used to reproduce observed vibrational frequencies and to compute molecular structures.
’ EXPERIMENTAL AND COMPUTATIONAL METHODS The experiment for reactions of laser-ablated metal atoms with hydrogen has been described in detail previously.18,19 The Nd: YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a rotating vanadium, niobium, or tantalum target (Johnson Matthey). Laser ablated metal atoms were codeposited with pure hydrogen or hydrogen (14%) in excess neon or argon onto a 4 K CsI cryogenic window at 24 mmol/h for 1 h. Isotopic D2 (Liquid Carbonic), HD (Cambridge Isotopic Laboratories), and selected mixtures were used in different experiments. The laser energy was varied from 10 to 20 mJ/pulse. Reducing the heat radiation from the ablation plume as low as possible keeps the solid H2 from melting. FTIR spectra were recorded at 0.5 cm1 resolution on Nicolet 750 with 0.1 cm1 accuracy in the 4500400 region using an MCTB detector. Matrix samples were annealed at different temperatures, and selected samples were subjected to broadband (wavelengths 220 nm to near-infrared) photolysis by a medium pressure mercury arc street lamp (Phillips, 175 W) with the globe removed. DFT (density functional theory) calculations of metal hydrides and hydrogen complexes are given for comparison. The Gaussian 09 program20 was employed to calculate the structures and frequencies of expected molecules. The 6-311+G(3df,3pd) basis set for H and V atoms and SDD pseudopotentials for niobium and tantalum were used.21,22 All the geometric parameters were fully optimized with B3LYP functionals23 and coupled-cluster methods (CCSD), and single point CCSD(T) Received: August 8, 2011 Revised: October 19, 2011 Published: October 24, 2011 14175
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Table 1. Infrared Absorptions (cm1) Observed from Reaction of Vanadium and Dihydrogen in Solid Neon, Argon, and Hydrogen neon H2
argon
HD
hydrogen
D2
H2
1553.9
1126.1
1564.1
1123.4
1503.9
1094.3
1516.4
1098.4
1543.1
HD
D2
H2
HD
D2
1492.2
1494.7
1087.2
VH2, VD2
1112.2
1492.9
1086.1
1106.1
1508.2
1092
1486.1
VH2(H2), VD2(D2) VHD(HD)
1087.7 1531.7
1541.3
1112.7
identification
VHD(HD)
1486.7
1080.2
VH2,
1082.0
VD2
1531.2
1536.2
1525.9
VHD
1107.8
1108.1
1101.6
VHD
Table 2. Infrared Absorptions (cm1) Observed from Reaction of Niobium and Dihydrogen in Solid Neon, Argon, and Hydrogen neon
argon
H2
HD
D2
H2
1757.6
1749.0, 1769.9, 1775.0, 1755.0
1263.5
1708.0 1705.2
1746.3
1259.3, 1265.7, 1268.7, 1263.2
1255.9
1677.4
1669.1
1662.7
1203.2
hydrogen
HD
D2
H2
D2
identification
1213.9, 1218.6, 1224.7
1225.2 1223.2
1754.8
1263.0
NbH4 (site) NbH4
1683.9
1679.1, 1694.7, 1704.5, 1707.3
1212.6
1747.5
1254.0
NbH4
1206.8
1610.4
1589.7
1154.0
1198.1
1569.0
1141.1
1128.2
1688.4
1214.7
NbH4 (site) NbH2 NbH2
518.6
energy calculations were performed.24 Analytical vibrational frequencies were obtained at the optimized structures.
’ RESULTS Infrared spectra are presented for V, Nb, and Ta atom reactions with H2 in excess neon, argon, and H2, and absorptions of reaction products are listed in Tables 1, 2, and 3. Absorptions common in experiments, namely, H(H2)n, (H,D)(HD)n, and D(D2)n, H(H2)n,25 (H,D)(HD)n, and D(D2)n, D3+(D2)n,26 and vibrational frequencies for normal H2 (75% ortho-H2 and 25% para-H2) and normal D2 (66% ortho-D2 and 33% para-D2),27,28 have been reported previously and are not discussed here. Trace impurity CO2, CO, H2O, and CH4 bands appeared in all experiments, but no impurity reaction products were observed with these molecules. Theoretical calculations are used to support the product identifications. Infrared Spectra. V + H2. Laser-ablated vanadium atoms were codeposited with H2 in neon, and infrared spectra of the reaction products are shown in (Figure 1). A strong and broad band centered at 1492.9 cm1 and a weak band at 1553.9 cm1 were observed in the VH stretching region after deposition. Irradiation >290 nm increased the 1492.9 cm1 absorptions by 2-fold. Annealing to 9 K produces a new band at 1503.9 cm1 and
NbH2 1707.6
1234.2
NbH4(H2)4
1705.3 1651.8
1228.8 1199.4
NbH4(H2)4 NbH4(H2)4
1646.6
1195.7
NbH4(H2)4
1635.0
1186.9
NbH3(H2)x
1615.5
1166.9
NbH3(H2)x
further annealing to 13 K increases this 1503.9 cm1 band markedly as a major stable product while the 1492.9 cm1 band decreased. All the isotopic D2 and HD counterpart bands in solid neon are listed in Table 1. For V atom reactions with H2 condensed to the solid, a strong band appeared at 1492.2 cm1 and a weak band at 1486.1 cm1 on codeposition (Figure 2). The 1486.1 cm1 band decreased on >290 nm irradiation, and disappeared on broad band irradiation. However the 1492.9 cm1 band increased on broad band irradiation. For V atom reaction products in solid D2 two counterpart bands were found at 1087.2 and 1080.2 cm1 (Figure 2b). The V atom reaction product spectra in solid HD are also shown in Figure 2. Furthermore, experiments in solid argon were done for comparison (not shown), and the product absorptions are listed Table 1. Nb + H2. Figure 3 shows reaction products of niobium atoms with H2 in solid neon, and new product absorptions for NbH stretching frequencies appeared at 1662.7, 1677.4, 1746.3, and 1757.6 cm1. These bands increased slightly on annealing to 8 K, decreased on full-arc irradiation, and recovered on annealing to 9 K. With D2 in neon these bands shift to the NbD stretching region as listed in Table 2, and HD reaction products are illustrated in the same table. The reaction products in solid normal H2, D2, and HD are quite simple: six new bands were observed at 1615.5, 1635.0, 1646.4, 1651.6, 1705.3, and 1707.6 cm1 in solid H2 and at 14176
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Table 3. Infrared Absorptions (cm1) Observed from Reaction of Tantalum and Dihydrogen in Solid Neon and Hydrogen neon
hydrogen
H2
HD
D2
H2
D2
identification
1841.5
1844.2, 1328.9
1322.4
TaH4
1301.0
TaH4
1240.6 1231.9
TaH2 TaH2
1840.6, 1324.0 1816.8
1816.2, 1302.8 1825.3, 1314.8
1712.8 1704.3
1708.0, 1234.9
1773.9
1274.3
TaH4(H2)4
1756.4
1262.9
TaH4(H2)4
1710.4
1231.3
TaH4(H2)4
1702.7
1225.6
TaH4(H2)4
1695.3
1221.9
TaH3(H2)x
1687.9
1214.8
TaH3(H2)x
Figure 1. (A) Infrared spectra for the vanadium atom and H2 reaction products in neon at 4.5 K: (a) V + H2 (2%) deposition for 60 min; (b) after >290 nm irradiation; (c) after annealing to 9 K; (d) after annealing to 13 K; (e) V + HD (4%) deposition for 60 min; (f) after >290 nm irradiation; (g) after annealing to 9 K; (h) after annealing to 13K. (B) Infrared spectra for the vanadium atom and D2 reaction products in neon at 4.5 K: (a) V + D2 (4%) deposition for 60 min; (b) after 240380 nm irradiation; (c) after annealing to 11 K; (d) after annealing to 13 K;. (e) V + HD (4%) deposition for 60 min; (f) after >290 nm irradiation; (g) after annealing to 9 K; (h) after annealing to 13 K.
1166.9, 1186.9, 1195.7, 1199.4, 1228.8, and 1234.2 cm1 in solid D2. These bands were not changed on further annealing and irradiation (Figure 5). The product absorptions in solid argon listed in Table 2 show consistent red shifts, and one NbH bending vibration was observed at 518.6 cm1. Ta + H2. The reaction product absorptions of tantalum with H2 in solid neon and pure H2 are very similar to that of niobium with H2, but all observed bands shifted blue about 100 cm1, and all the absorptions are shown in Figures 4 and 5 and listed in Table 3. However in solid argon the reaction products of Ta with H2 are much weaker than those of Nb + H2. Calculations. The structures of the MH2, MH4, and MH4(H2)14 (M = V, Nb, Ta) species have been optimized at the DFT and CCSD levels of theory, the bond distances and angles are reported in Figure 6, and the calculated harmonic vibrational frequencies are reported in Tables 46. As H2 moieties associate with VH2, NbH2, and TaH2, the high-spin hydrogen complex
Figure 2. (A) Infrared spectra for the vanadium atom and H2 reaction products in pure H2 at 4.5 K: (a) V + H2 deposition for 30 min; (b) after 240380 nm irradiation; (c) after full-arc irradiation; (d) V + HD deposition for 30 min; (e) after 240380 nm irradiation; (f) after annealing to 8 K. (B) Infrared spectra for the vanadium atom and D2 reaction products in pure D2 at 4.5 K: (a) V + D2 deposition for 30 min; (b) after full-arc irradiation; (c) after annealing to 9 K; (d) V + HD deposition for 30 min; (e) after 240380 nm irradiation; (f) after annealing to 8 K.
VH2(H2) while low-spin hydride NbH4 and TaH4 are lowest in energy. The same periodic trend is observed in Cr, Mo, and W + H2, where CrH2(H2), MoH4, and WH4 are identified from infrared spectra and theoretical calculations.2932 The H2 moieties can be coordinated to NbH4 and TaH4 to give NbH4(H2) and TaH4(H2), which further combine more H2 giving NbH4(H2)14 and TaH4(H2)14. However no higher vanadium polyhydrogen complexes are given in the same calculation. Energy levels for niobium and tantalum tetrahydrides and dihydrogen complexes calculated at the B3LYP level of theory and single-point energy calculations at CCSD(T) are shown in Figure 7. An estimate of the basis-set superposition correction is less than 1 kcal/mol for metal hydride DFT calculations.29
’ DISCUSSION New infrared absorptions will be assigned to tantalum, niobium, and vanadium dihydrides, tetrahydrometalate and hydrogen complexes based on the isotopic distributions of D2, H2 + D2, and HD substitutions, comparison of neon, argon, and solid H2, D2, and HD, and agreements with DFT frequency calculations. VH2 and VH2(H2). As shown in (Figure 1) a broad band centered at 1492.9 cm1 appeared on initial codeposition of vanadium atoms with H2 in excess neon at 4 K, which increased 2-fold with 290380 nm irradiation applied to the sample. Annealing to 11 K produced one sharp band at 1492.9 cm1 and one weak band at 1569.4 cm1. The deuterium counterparts are found at 1086.1 and 1129.8 cm1, giving 1.3746 and 1.3891 H/D isotopic frequency ratios, respectively. Experiments with HD give new intermediate bands at 1531.2 cm1 in the upper region and at 1107.8 in the lower region, which are appropriate for VH antisymmetric and symmetric modes of isolated VH2. These neon matrix absorptions are weaker than the stronger argon matrix bands assigned below. In addition the stronger neon matrix absorption at 1492.9 cm1 is lower in frequency than the stronger argon matrix absorption at 1502.8 cm1. This is not the usual relationship for neon and argon matrix frequencies,33 which might indicate that this neon matrix species is not isolated but is a complex of VH2. The VH2 species was also trapped in our solid argon experiments. Two bands at 1508.2 cm1 (stronger) and 1564.1 cm1 (weaker) tracked together on deposition of V with H2 in argon, increased by 10% on annealing to 20 K, and further increased 2-fold on >290 nm irradiation. The deuterium counterparts were 14177
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Figure 3. (A) Infrared spectra for the niobium atom and H2 reaction products in neon at 4.5 K: (a) Nb + H2 (2%) deposition for 60 min; (b) after annealing to 8 K; (c) after full-arc irradiation; (d) after annealing to 9 K; (e) Nb + HD (4%) deposition for 60 min; (f) after annealing to 8 K; (g) after full-arc irradiation; (h) after annealing to 11 K. (B) Infrared spectra for the niobium atom and D2 reaction products in neon at 4.5 K: (a) Nb + D2 (2%) deposition for 60 min; (b) after annealing to 8 K; (c) after full-arc irradiation; (d) after annealing to 9 K; (e) Nb + HD (4%) deposition for 60 min; (f) after annealing to 8 K; (g) after full-arc irradiation; (h) after annealing to 11 K.
Figure 4. (A) Infrared spectra for the tantalum atom and H2 reaction products in neon at 4.5 K: (a) Ta + H2 (2%) deposition for 60 min; (b) after annealing to 8 K; (c) after full-arc irradiation; (d) after annealing to 9 K; (e) Ta + HD (4%) deposition for 60 min; (f) after annealing to 8 K; (g) after fullarc irradiation; (h) after annealing to 11 K. (B) Infrared spectra for the tantalum atom and D2 reaction products in neon at 4.5 K: (a) Ta + D2 (2%) deposition for 60 min; (b) after annealing to 8 K; (c) after full-arc irradiation; (d) after annealing to 9 K; (e) Ta + HD (4%) deposition for 60 min; (f) after annealing to 8 K; (g) after full-arc irradiation; (h) after annealing to 11 K.
Figure 5. Infrared spectra for the niobium atom and H2 reaction products in pure H2 at 4.5 K: (a) Nb + H2 deposition for 30 min; (b) annealing to 6 K; (c) after >380 nm irradiation; (d) after annealing to 6.4 K. Infrared spectra for the tantalum atom and H2 reaction products in pure H2 at 4.5 K: (e) Ta + H2 deposition for 30 min; (f) annealing to 6 K; (g) after >380 nm irradiation; (h) after annealing to 6.4 K.
found at 1092.0 and 1123.4 cm1. Intermediate bands at 1536.2 cm1 (VH stretching region) and at 1108.1 cm1
Figure 6. Structures of vanadium, niobium, and tantalum hydrides and dihydrogen complexes calculated at the B3LYP and CCSD levels of theory. CCSD values are in bold italic.
(VD stretching region) were obtained with HD. Vanadium atom reactions with H2 have been investigated in solid argon by 14178
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Table 4. Observed and Calculated Frequencies (cm1) for VH2(H2)a VH2(H2)
obsd (Ne) obsd (Ar)
1503.9
1516.4
calcd
calcd
(B3LYP)
(CCSD)
3982.7(a0 , 60)b 1657.9(a0 , 139)
VHD(HD)
calcd
calcd
(B3LYP)
(CCSD)
4060.3(31)
2817.3(34)
2872.2(15)
1660.3(109)
1179.1(73)
1179.1(58)
1616.1(a00 , 639) 1590.9(802)
obsd (Ne) obsd (Ar)
1094.3
1098.4
1160.8(332) 1144.5(413)
obsd (Ne) obsd (Ar)
calcd
calcd
(B3LYP)
(CCSD)
3451.0(52)
3518.1(24)
1543.1
1541.3
1636.9(376) 1626.1(422)
1112.7
1112.2
1170.7(217) 1162.2(268)
1104.6(a00 , 6)
1108.7(6)
782.1(4)
784.9(4)
972.3(9)
615.0(a0 , 8)
605.6(1)
444.8(1)
438.1(0)
502.7(6)
492.5(1)
535.1(a0 ,150)
461.2(5)
385.4(78)
326.7(3)
465.9(118)
400.0(38)
459.9(a00 , 11) 357.0(a0 , 346)
399.0(349) 387.1(331)
325.9(6) 254.8(178)
287.9(172) 276.3(177)
394.6(24) 307.7(247)
347.5(255) 336.8(228)
17.5(a00 , 5) a
VD2(D2)
196.1(6)
11.3(2)
139.5(3)
17.3(4)
975.6(8)
159.7(8)
Basis set: H: 6-311++G(3df,3pd); V: SDD. b (mode symmetries, computed intensities, km/mol).
the Rice group.14 The weaker assigned 1532.4 cm1 symmetric stretching mode for VH2 is not consistent with our 1564.1 cm1 value nor the 1536 cm1 VHD assignments of both groups; however, the stronger assigned VH antisymmetric mode at 1508.2 cm1 is correct. Hence, their 131 ( 5° angle calculation based on the VH mode intensities is not correct. Although their krypton matrix assignments appear to be correct, their 12 K substrate temperature is not cold enough for effective isolation of metalhydrogen reaction products in solid argon, based on our considerable experience with hydrogen matrix systems. We used a 4 K substrate for all of our matrix experiments. In the solid neon matrix a sharp band at 1503.9 cm1 was observed after annealing to 9 K while the bands due to VH2 at 1492.9 and 1553.9 cm1 decreased. Apparently the increase of this new species on annealing is at the expense of VH2, suggesting VH2 is coordinated by H2 to form the VH2(H2) complex in H2/ Ne experiments. The 1503.9 cm1 band can be assigned to the antisymmetric VH stretching mode of VH2 moiety in the VH2(H2) complex. Unfortunately the symmetric VH mode was not observed because of band weakness. With D2 in solid neon the counterpart band appeared at 1094.3 cm1 and increased on annealing to 9 K by D2 coordinating to VD2 to form VD2(D2). With HD in neon substituted bands at 1543.1 cm1 in the VH stretching region and 1112.7 cm1 in the VD stretching region confirm the VH2(H2) assignment. The assignments of VH2(H2) are strongly supported by solid H2 and D2 experiments. As laser-ablated V atoms were cocondensed with pure hydrogen, H2 acts as both matrix and reagent.12 A strong and sharp band at 1492.2 cm1 and a weak band at 1486.1 cm1 are the major product absorptions. Note the 1486.1 cm1 band, which is appropriate for VH2, disappeared on sample annealing to 6.6 K while the 1492.2 cm1 band survived at last (Figure 2), suggesting the reaction of VH2 with H2 gave VH2(H2). In pure D2 two bands were observed at 1087.2 and 1080.2 cm1, respectively, which are due to VD2(D2) and VD2. With pure HD new bands at 1531.7 and 1525.9 cm1 in the VH stretching region and at 1106.1 and 1101.6 cm1 in the VD stretching region were generated for the VHD(HD) and VHD molecules in addition to weaker absorptions of VH2, VH2(H2), VD2, and VD2(D2). Notice that the yields of VH2(H2) and VD2(D2) were dominant in solid H2 and D2 because of abundant H2 and D2 in pure solid reactions, respectively. Similar experiments were done in solid argon, and the strong absorption of VH2(H2) was observed at 1516.4 cm1, which was
observed very weakly on deposition and increased 3-fold on annealing to 25 K. The absorption of VD2(D2) was found at 1098.4 cm1, and the bands due to VHD(HD) were identified at 1541.3 and 1112.2 cm1, which correlated with neon and hydrogen experiments very well. Note the yield of VH2(H2) in solid argon is much less than that obtained in solid neon and pure hydrogen because the more polarizable argon medium inhibits the combination of VH2 with H2. Theoretical calculations at the B3LYP level predicted the (H2)VH2 complex with a high-spin quartet ground state and Cs symmetry while the low-spin doublet VH4 with D2d symmetry was located 16 kcal/mol higher in energy, which is in good agreement with the higher level calculations of the Schaefer group, who first predicted the (H2)VH2 complex to be more stable than VH4.16 Our calculated frequencies gave a strong antisymmetric VH stretching mode for VH2(H2) at 1616.1 cm1, which is overestimated by 6.3% for the neon value and 5.4% for the argon value, in line with harmonic DFT frequency calculation for transition metal hydrides.19b With our CCSD calculation this mode was predicted at 1590.9 cm1, only about 3% higher than experimental values (Table 4). Similar calculated results were reported at the CISD level for VH2 at 1597 cm1 and VH2(H2) at 1600 cm1.16 NbH2 and NbH4. The absorptions of NbH2 in neon were found for the NbH antisymmetric and symmetric modes at 1662.8 and 1677.4 cm1, which shift to 1198.2 and 1206.7 cm1, respectively, in the deuterium experiment. With HD median bands at 1669.1 cm1 in the NbH stretching region and at 1203.2 cm1 in the NbD stretching region were observed. A stronger band at 1569.0 cm1 and a weaker band at 1610.4 cm1 track the 518.6 cm1 band on the reaction of Nb atom with H2 in solid argon. These are assigned to the symmetric and antisymmetric stretching and bending modes, respectively, for NbH2 also in agreement with the results obtained by Van Zee et al.15 The absorptions of NbH4 appeared at 1757.2 and 1746.8 cm1 in solid neon, which are assigned to symmetric and antisymmetric NbH stretching vibrations. This pair of bands slightly increased on annealing and >290 nm irradiation, and a very weak band at 490.1 cm1 in the HNbH deformation region tracked with the 1757.2 and 1746.8 cm1 bands. With D2 in neon two NbD stretching modes shifted to 1264.0 and 1256.3 cm1, respectively, defining 1.3911 and 1.3905 H/D isotopic frequency ratios. However the DNbD deformation mode shifted out of our 14179
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397.7(76) 419.4(115)
354.3(103)
464.1(143) 485.5(151)
584.8(57)
547.3(2)
1291.4(99)
1285.6(203) 1218.9
334.9(84) a
673.8(0)
325.9i(0)
529.8(213 2)
468.2(185)
637.1(a1, 0)
621.0(b1, 0)
521.8(e, 177 2)
422.5(b2, 150)
1705.2
1683.9
490.1
1757.2
1746.8
518.6
Basis set: H, 6-311++G(3df,3pd); Nb, SDD. b (mode symmetries, computed intensities, km/mol).
335.2(94) 302.6(76)
379.5(109 2) 373.8(90 2)
572.1(29) 476.6(0)
230.0i(0) 439.3(0)
450.7(0)
537.9(0)
1263.2
1265.7
1213.9 1259.4
1298.2(118) 1224.7 1269.0 1298.9(232 2) 1278.9(176 2) 1214.7 1821.7(422 2) 1794.4(e, 336 2)
1256.3
1223.2 1890.3(0) 1841.5(566) b
1844.5(a1, 0) 1811.6(b2, 421)
1264.0
obsd (Ne) calcd
CCSD obsd (Ar)
calcd
ARTICLE
obsd (Ne)
B3LYP
1279.9(186)
1819.9(158) 1802.9(374) 1707.3 1704.5 1769.7 1755.0 1828.2(202) 1793.8(328) 1694.7 1679.1 1775.0 1749.3 1337.2(0) 1313.3(295) 1304.7(0) 1291.7(219)
CCSD
calcd calcd
B3LYP obsd (Ar)
NbD4 NbH4
Table 5. Observed and Calculated (B3LYP/6-311++G(3df,3pd)/SDD) Frequencies for NbH4 (2B1)a
obsd (Ne)
obsd (Ar)
NbH2D2
calcd B3LYP
obsd (Ne)
obsd (Ar)
NbH2D20
calcd B3LYP
The Journal of Physical Chemistry A
measurement region. The HD experiments gave diagnostic quartet distributions in both NbH and NbD stretching regions: two stronger absorptions at 1769.7 and 1749.3 cm1 with two weaker bands at 1775.0 and 1755.0 cm1 in the NbH stretching region, and two stronger bands at 1265.7 and 1259.4 cm1 with weaker bands at 1269.0 and 1263.2 cm1 in the NbD stretching region. This isotopic pattern of metal hydrogen stretching modes is different from metal tetrahydride absorptions we obtained before for ZrH4, HfH4, TiH4, WH4, PbH4, and MH4 (M = Sc,Y, and La) with Td symmetry and AuH4 with D4h symmetry.9,30 A different symmetry structure for NbH4 must be considered here. On the basis of both B3LYP and CCSD calculations the ground state structure of NbH4 converged to D2d symmetry instead of a Td structure. The predicted HNbH bond angles for the 2B1 electronic state were distorted to 105.0° and 111.8° (B3LYP) and 105.4° and 111.6° (CCSD), respectively. For the 2 A1 electronic state the distortion of HNbH angles goes a little more to 107.7° and 113.0° (B3LYP) and 107.3° and 113.8° (CCSD). As a result the degenerate t2 mode for Td symmetry is split into b2 and degenerate e modes. The calculated frequencies for the 2B1 electronic state match the experimental values better than that for 2A1 state although the two electronic states have nearly identical energies. First, the B3LYP frequency calculation gave NbH stretching modes for NbH4 at 1811.3 cm1 (b2) and 1794.3 cm1 (e), which are overestimated only by about 3% for both modes in neon and by about 6% for both modes in argon, and the deviations are in excellent agreement with that for NbH2. In addition the B3LYP calculation predicted the degenerate deformation mode at 511.7 cm1, which is 21 cm1 higher than that observed and in satisfactory agreement. Second, the calculated intensity of the e mode for the 2B1 state is twice as strong as the b2 mode, which is in good agreement with observation. Third, with HD substitution the calculated frequencies reproduce experimental bands very well; both quartets are due to symmetric and antisymmetric NbH2, NbD2, and NbHD stretching fundamentals in the NbH2D2 molecule as symmetry is lowered to C2v. Similar absorptions due to NbH4 in solid argon were observed at 1705.2 (site 1708.0) cm1 and 1683.9 (site 1688.4) cm1 on deposition, and the matrix sites merged to 1705.2 and 1683.9 cm1 upon annealing. The D counterparts were found at 1223.2 (site 1225.2) cm1 and 1212.6 (site 1214.7) cm1. With HD absorptions at 1705.0, 1707.6, 1725.2, and 1728.1 cm1 in the NbH stretching region and at 1213.7, 1217.8, 1234.2, and 1238.3 cm1 in NbD stretching region, which are in very good accord with neon isotopic distributions. In solid hydrogen very weak bands at 1754.8 and 1747.5 cm1 are appropriate for NbH4, and NbD4 bands are located at 1263.0 and 1254.0 cm1 in solid deuterium. NbH4(H2)4. Absorptions at 1705.3 and 1646.6 cm1 (matrix sites at 1707.6 and 1651.8 cm1) were observed as major product bands in laser-ablated Nb atom reaction with H2 in solid hydrogen. These absorptions are appropriate for the NbH4(H2)4 complex based on the following reasons. First, the deuterium counterpart bands shift to 1228.8 and 1195.7 cm1, giving 1.387 H/D isotopic frequency ratios, which is the typical ratio for NbH stretching modes. Second, the NbH4 molecule has been identified with D2d symmetry in solid neon (1757.6 and 1746.3 cm1) and argon (1705.2 and 1683.9 cm1), and the weak new band at 1754.8 cm1 in solid hydrogen is due to NbH4 trapped on the surface just like WH4 was observed on the surface of solid hydrogen.29 Notice that the NbH stretching mode in NbH4(H2)4 is located 50 cm1 lower than in NbH4, which is 14180
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Table 6. Observed and Calculated (B3LYP/6-311++G(3df,3pd)/SDD) Frequencies for TaH4 (2A1)a TaH4
obsd (Ne)
TaD4 calcd
calcd
(B3LYP)
(CCSD)
1884.9(a1, 0)b
a
obsd (Ne)
1927.6(0)
calcd
calcd
(B3LYP)
(CCSD)
1333.3(0)
calcd obsd (Ne)
1363.5(0)
1841.5
1844.0(e,387 2) 1887.5(512 2)
1322.4
1309.9(198 2) 1341.2(263 2)
1816.8
1834.4(b2, 228)
1301.0
1302.8(117)
1872.9(228)
TaH2D20
TaH2D2
1330.2(117)
(B3LYP)
calcd obsd (Ne)
(B3LYP)
1844.2
1860.2(106)
1840.6
1864.7(183)
1816.2
1843.6(380)
1825.3
1838.3(300)
1328.9
1317.7(67)
1324.0
1321.7(111)
1302.8
1310.5(205)
1314.8
1307.8(166)
936.1(b1, 0)
648.5(0)
662.2(0)
458.7(0)
810.7(0)
821.6(15)
622.0(a1, 0)
590.5(0)
439.9(0)
417.7(0)
596.9(52)
545.5(9)
566.1(b2, 130)
612.4(152)
403.2(66)
436.1(77)
439.5(129) 418.7(45)
501.9(108) 391.7(99)
467.0(e,147 2)
469.5(193 2)
332.5(74 2)
334.2(98 2)
367.8(92)
372.8(86)
Basis set: H, 6-311++G(3df,3pd); Ta, SDD. b (mode symmetries, computed intensities, km/mol).
Figure 7. Energy level diagram for niobium and tantalum hydrides and dihydrogen complexes calculated at the B3LYP structure with single-point energy calculations using the CCSD(T) level of theory.
exactly the same case for WH4 and WH4(H2)4, where the difference in WH stretching modes for both molecules is about 50 cm1. Third, our DFT calculations support this assignment. The calculated NbH4(H2)4 species is the most stable complex for NbH4 coordinated by H2 (see Figure 7) and any more H2 ligand addition to NbH4(H2)4 is endothermic. With the B3LYP functional the predicted antisymmetric degenerate (e) mode at 1692.6 cm1 is overestimated by 46 cm1 and antisymmetric combination of symmetric (b2) HNbH stretching modes at 1707.5 cm1 is essentially the same as observed the values in solid hydrogen. Notice the calculated HH stretching, Nb(H2) stretching and HNbH bending modes are extremely weak, which is in agreement with our experiments in that no more absorptions were observed for this complex. The experiments with solid HD have also been done, but the very strong niobium hydrogen and very weak niobiumdeuterium stretching frequencies dominated because of isotopic exchanges between the Nb bonded H and D atoms and the complexed HD molecules . TaH2 and TaH4. A very strong band at 1704.3 cm1 is attributed to the antisymmetric stretching mode of TaH2 while
the weak symmetric mode is found at 1712.8 cm1. The deuterium counterpart was found at 1231.9 and 1240.6 cm1. With HD, slightly broad features centered at 1708.0 and 1234.9 cm1 were obtained, which are the bands due to TaHD. The analogous tetrahedral tantalum hydride TaH4 was identified in the reaction of laser-ablated Ta atoms with molecular hydrogen. In solid neon two bands at 1841.4 and 1816.6 cm1 tracked together, which appeared on deposition and increased on annealing. The deuterium substituted bands shifted to 1322.4 and 1301.0 cm1. With HD four upper bands were found at 1844.2, 1840.6, 1816.2, and 1825.3 cm1 in the TaH stretching region and four lower bands appeared at 1328.9, 1324.0, 1302.8, and 1314.8 cm1 in the TaD stretching region. The observed isotopic distribution is appropriate for TaH4, which is analogous to NbH4. However the observed 1841.4 cm1 band is much stronger than the 1816.8 cm1 band, suggesting that the degenerate e mode is located higher than the b2 mode, which is appropriate for the 2A1 electronic state. Both B3LYP and CCSD calculations gave identical 2B1 and 2A1 electronic state energies, but the calculated frequencies for the 2A1 14181
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state match the experimental values better. At the B3LYP level the calculated frequencies gave the stronger degenerate e mode at 1844.0 cm1 and the weaker b2 mode at 1834.4 cm1, which are only overestimated by about 1%. Notice that the calculated band splitting pattern for TaH2D2 is similar to the niobium case. TaH4(H2)4. One strong absorption at 1756.4 cm1 tracking two weak bands at 1710.4 and 1702.7 cm1 in solid hydrogen are appropriate for the TaH4(H2)4 complex. Notice the 1756.4 cm1 band is much stronger than the 1710.4 and 1702.7 cm1 bands, suggesting the symmetry for the TaH4 core in the TaH4(H2)4 complex is similar to that for the isolated TaH4 molecule in solid neon. In solid deuterium these bands shift to 1262.9, 1231.3, and 1225.6 cm1, respectively, giving approximately 1.39 H/D isotopic frequency ratios. This assignment is supported by DFT and CCSD calculations. With the B3LYP functional and CCSD the maximum number of H2 molecules that can be attached to TaH4 is four, which is the same as for the niobium case. The calculated TaH stretching frequencies (B3LYP) with IR active absorption for TaH4(H2)4 were predicted at 1746.9 cm1 (stronger) and 1732.0 and 1728.8 cm1 (weaker), which match the observed values very well. Reaction Mechanisms and Bonding. The insertions (reaction 1) of ground V, Nb, and Ta, into H2 are exothermic by 18, 24, and 15 kcal/mol, respectively, calculated at the B3LYP level, which is in agreement with the CCSD predication. M þ H2 f MH2
ð1Þ
This reaction is not spontaneous for the V, Nb, and Ta metals in the low temperature matrix environment. This is consistent with theoretical calculations, which suggested that ground state Ta(4F) has a 24 kcal/mol energy barrier to insert H2 and form TaH2.34 This is why the Rice group had to irradiate cold thermally deposited V atoms at 320380 nm to initiate the reaction with dihydrogen.14 In the case of laser ablation, both excited metal atoms and metal resonance radiation are produced.19 The NbH2 and TaH2 intermediates further react with H2 to give NbH4 and TaH4, which are spontaneous without any significant activation energy requirement in the low temperature matrix (reaction 2), and these reactions are exothermic by 27 (Nb) and 45 (Ta) kcal/mol, respectively. In contrast the reaction of VH2 with H2 gives the complex VH2(H2) (reaction 3), which is exothermic only by 4 kcal/mol. A similar chemistry trend was observed for group 6 transition metals where the complex CrH2(H2) and the hydrides MoH4 and WH4 were identified in low temperature matrices.32 MH2 þ H2 f MH4
ð2Þ
VH2 þ H2 f VH2 ðH2 Þ
ð3Þ
The NbH4 and TaH4 molecules combine nH2 molecules to form the NbH4(H2)n and TaH4(H2)n complexes (n = 1, 2, 3, 4) (reaction 4), which are exothermic by 9, 23, 30, and 44 kcal/mol for Nb and 10, 23, 32, and 43 kcal/mol for Ta calculated at the B3LYP minima using the CCSD(T) method. Calculations predict that NbH4(H2)4 and TaH4(H2)4 are the maximum sized stable complexes. However no additional reaction was observed for VH2(H2), and calculations suggest that VH2(H2)2 is not stable on the potential energy surface. MH4 þ nH2 f MH4 ðH2 Þn ðM ¼ Nb, Ta; n ¼ 1, 2, 3, 4Þ
ð4Þ
’ CONCLUSIONS Laser ablated V, Nb, and Ta atoms react with H2 during codeposition at 4 K in excess neon, argon, and hydrogen to give the MH2 molecules, which react further with H2 to form the complex VH2(H2) and the tetrahydrides NbH4 and TaH4 with D2d symmetry. These lower symmetry structures were confirmed by the observation of mixed isotopic quartets in both NbH and NbD stretching regions for NbH2D2. The latter are different from most transition metal tetrahydrides with Td symmetry. Supporting DFT and CCSD theoretical calculations were used to predict energies, geometries, and vibrational frequencies of these novel metal hydrides. Higher hydrogen complexes NbH4(H2)4 and TaH4(H2)4 are found for the NbH4 and TaH4 molecules upon further reaction with H2 in solid hydrogen. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge support for this work from NCSA computing Grant (CHE07-0004N) to L.A. X.W. is grateful for the support from NNSFC (20973126) and STCSM (10PJ1409600). ’ REFERENCES (1) Dresselhaus, M. S.; Thoms, I. L. Nature 2001, 414, 332–337. (2) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353–358. (3) Hinrichs, R. Z.; Schroden, J. J.; Davis, H. F. J. Phys. Chem. A 2003, 107, 9284–9294. (4) Gandelman, M.; Shimon, L. J. W.; Milstein, D. Chem.—Eur. J. 2003, 9, 4295–4300. (5) Lee, D. Y.; Kim, I. J.; Jun, C. H. Angew. Chem., Int. Ed. 2002, 41, 3031–3033. (6) Capitano, A. T.; Gland, J. L. J. Phys. Chem. B 1998, 102, 2562–2568. (7) Rankin, M. A.; Cummins, C. C. J. Am. Chem. Soc. 2010, 132, 10021–10023. (8) (a) Andrews, L.; Wang, X. F. Science 2003, 299, 2049–2052. (b) Wang, X. F.; Andrews, L.; Tam, S.; DeRose, M. E.; Fajardo, M. E. J. Am. Chem. Soc. 2003, 125, 92189228. (Al2H6). (9) (a) Wang, X. F.; Andrews, L., J. Am. Chem. Soc. 2002, 124, 76107613 (ScH4). (b) Wang, X. F.; Andrews, L. J. Am. Chem. Soc. 2001, 123, 1289912900 (AuH3). (c) Chertihin, G. V.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 83228327 (Ti + H2). (d) Chertihin, G. V.; Andrews, L. J. Am. Chem. Soc. 1995, 117, 6402-6403 (Zr, Hf + H2). (e) Wang, X. F.; Andrews, L., J. Am. Chem. Soc. 2003, 125 6581-6587 (Pb + H2). (10) Wang, X. F.; Andrews, L. J. Phys. Chem. A 2003, 107, 4081 4091 (Mn, Reaction + H2). (11) Wang, X. F.; Andrews, L. J. Phys. Chem. A 2002, 106, 3706 3713. (Rh + H2). (12) (a) Andrews, L. Chem. Soc. Rev. 2004, 33, 123–132. (b) Wang, X.; Andrews, L.; Infante, I.; Gagliardi, L. J. Phys. Chem. A 2009, 113, 1256612572 (Ln + H2). (c) Raab, J.; Lindh, R. H.; Wang, X. F.; Andrews, L. Gagliardi, L. J. Phys. Chem. A 2007, 111, 63836387 (U + H2). (13) (a) Wang, X.; Andrews, L. Organometallics 2008, 27, 4273– 4276. (b) Wang, X.; Andrews, L. J. Phys. Chem. A 2009, 113, 551563 (Fe, Ru, Os + H2). (14) Xiao, Z .L.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. 1991, 95, 2696–2700. (15) Van Zee, R. J.; Li, S.; Weltner, W., Jr. J. Chem. Phys. 1995, 102, 4367–4374. 14182
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(16) Ma., B.; Collins, C. L.; Schaefer, H. F., III J. Am. Chem. Soc. 1996, 118, 870–879. (17) Balabanov, N. B.; Boggs, J. E. J. Phys. Chem. A 2000, 104, 1597–1601. (18) (a) Zhou, M.; Andrews, L. J. Phys. Chem. A 1998, 102, 8251–8260. (b) Chertihin, G. V.; Bare, W. D.; Andrews, L. J. Phys. Chem. A 1997, 101, 5090–5096. (19) (a) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040–4053 and references therein. (b) Andrews, L. Chem. Soc. Rev. 2004, 33, 123–132 and references therein. (20) 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, Jr., J. A.; 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, N. J.; 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, € Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; S.; Daniels, A. D.; Farkas, O.; Fox, D. J. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (21) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650–654. (22) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265–3269. (23) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.; Yang, E.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (24) (a) Purvis, G. D., III; Bartlett, R. J. J. Chem. Phys. 1982, 76, 1910–1918. (b) Scuseria, G. E.; Schaefer, H. F., III J. Chem. Phys. 1989, 90, 3700–3703. (c) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968–5975. (25) Andrews, L.; Wang, X. J. Phys. Chem. A 2004, 108, 3879–3883. (26) Wang, X.; Andrews, L. J. Phys. Chem. A 2004, 108, 1103–1106. (27) Gush, H. P.; Hare, W. F.; Allin, E. J.; Welsh, H. L. Can. J. Phys. 1960, 38, 176–193. (28) Crane, A.; Gush, H. P. Can. J. Phys. 1966, 44, 373–398. (29) Wang, X. F.; Andrews, L.; Infante, I.; Gagliardi, L. J. Am. Chem. Soc. 2008, 130, 19721978 (W + pure H2). (30) Wang, X. F.; Andrews, L. J. Am. Chem. Soc. 2002, 124, 56365637 (WH6 in neon). (31) Wang, X. F.; Andrews, L. J. Phys. Chem. A 2005, 109, 90219027 (Mo + H2). (32) Wang, X. F.; Andrews, L. J. Phys. Chem. A 2003, 107, 570578 (Cr + H2). (33) Jacox, M. E. Chem. Phys. 1994, 189, 149–170. (34) Dai, D. G.; Cheng, W.; Balasubramanian, K. J. Chem. Phys. 1991, 95, 9094–9105.
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