Article pubs.acs.org/IC
Matrix Infrared Spectra and Quantum Chemical Calculations of Ti, Zr, and Hf Dihydride Phosphinidene and Arsinidene Molecules Lester Andrews*,† and Han-Gook Cho†,‡ †
Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States Department of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon, 406-772, South Korea
‡
S Supporting Information *
ABSTRACT: Laser ablated Ti, Zr, and Hf atoms react with phosphine during condensation in excess argon or neon at 4 K to form metal hydride insertion phosphides (H2P-MH) and metal dihydride phosphinidenes (HPMH2) with metal phosphorus double bonds, which are characterized by their intense metal−hydride stretching frequencies. Both products are formed spontaneously on annealing the solid matrix samples, which suggests that both products are relaxed from the initial higher energy M-PH3 intermediate complex, which is not observed. B3LYP (DFT) calculations show that these phosphinidenes are strongly agostic with acute H−PM angles in the 60° range, even smaller than those for the analogous methylidenes (carbenes) (CH2MH2) and in contrast to the almost linear HNTi subunit in the imines (H-NTiH2). Comparison of calculated agostic and terminal bond lengths and covalent bond radii for HPTiH2 with computed bond lengths for Al2H6 finds that these strong agostic Ti−H bonds are 18% longer than single covalent bonds, and the bridged bonds in dialane are 10% longer than the terminal Al−H single bonds, which show that these agostic bonds can also be considered as bridged bonds. The analogous arsinidenes (HAsMH2) have 4° smaller agostic angles and almost the same metal−hydride stretching frequencies and double bond orders. Calculations with fixed H−P− Ti and H−As−Ti angles (170.0°) and Cs symmetry find that electronic energies increased by 36 and 44 kJ/mol, respectively, which provide estimates for the agostic/bridged bonding energies.
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calculations.14 Nucleophilic phosphinidene complexes including substitution at phosphorus, LnMP-R, have been reviewed.15 Terminal hafnium phosphinidene complexes have also been investigated.16 Both arsinidene and phosphinidene complexes have been formed using 1,2-H2 elimination from silox derivatives.17 Yittrium complexes of arsine, arsenide, and arsinidene ligands have been recently reported. 18 Reactions of tungstenphosphinidene and tungsten-arsinidene complexes with carbodiimides and alkyl azides have been investigated.19 We report here the simplest group 4 metal hydride parent phosphinidene molecules (HPMH2) from the direct reaction of metal atoms with phosphine. The analogous arsinidene complexes (HAsMH2) have also been prepared from arsine for comparison of structures and frequencies.
INTRODUCTION The parent phosphinidene, PH, has been investigated in the gas phase,1−3 and the vibrational frequency of 2276.0 cm−1 and equilibrium bond length of 1.42218 Å have been determined for the 3Σ− ground state from the vibration-rotation emission spectrum.2 Such a triplet spin state reagent would be useful to form a simple substituted double bonded phosphinidene, and reaction with the O atom leads directly to H-PO, which has been observed in the gas phase and inert matrix samples.4−6 It is, therefore, expected that transition metal phosphinidene molecules (HPTiH2) can be prepared like the simple methylidenes (CH2TiH2) produced from reactions of laser ablated group 4 metal atoms with methane7−9or imines from their reactions with ammonia.10,11 Previous MCSCF calculations on HNTiH2 and HPTiH2 predicted substantial triple bond character for the linear H-NTi imine moiety and a triangular strongly agostic H-PTi subunit for the phosphinidene.12 Although complexes of the parent P-H phosphinidene are scarce, there have been both theoretical and experimental investigations of substituted derivatives. A DFT investigation for a series of transition metal MLnPH phosphinidenes reported that their reactivity is influenced by the ligand, L, and that the metal−PH interaction increases on passing from the first to the second and third row transition metals.13 A carbene stabilized parent phosphinidene, H-P:L has been prepared and investigated by X-ray crystallography and quantum chemical © XXXX American Chemical Society
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EXPERIMENTAL AND COMPUTATIONAL METHODS
Laser ablated Ti, Zr, and Hf atoms were reacted with phosphine (Matheson, after condensation at 77 K to remove volatile impurities) diluted in argon during condensation at 4 K using a closed-cycle refrigerator (Sumitomo). Arsine (Matheson, 10% in hydrogen, condensed at 77 K to remove H2) was treated likewise. Deuterium exchanged phosphine was prepared by condensing PH3 on D2O Received: June 4, 2016
A
DOI: 10.1021/acs.inorgchem.6b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry several times in a D2O exchanged stainless steel vacuum line, which resulted in stronger PD3, PHD2, and PH2D than residual PH3 absorptions. Reagent gas mixtures were typically 1% or 2% in argon or neon (Spectra Gases). Caution! These reagents are highly toxic: our system pump was vented through a garden hose into a laboratory f ume hood exhausted through the roof. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused onto a rotating metal target (Johnson-Matthey) using 5−10 mJ/pulse. After initial reaction, infrared spectra were recorded at 0.5 cm−1 resolution using a Nicolet 750 spectrometer with a Hg-Cd-Te range B detector. Selected samples were irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed using a combination of optical filters or annealed to allow further reagent diffusion. These methods have been described in previous publications.7−9,20,21 In order to provide support for the assignment of new experimental frequencies and to correlate with related works,10,11 density functional theory (DFT) calculations were performed using the Gaussian 09 package,22 the B3LYP density functional,23 and the aug-cc-pVTZ basis sets for H, N, P, As, and Ti and aug-cc-pVTZ-pp basis sets and pseudo potentials for Zr and Hf.24 The 6-311++G(3df,3pd) basis set was used for comparison with titanium product calculations.25 Geometries were fully relaxed during optimization, and the optimized geometry and transition-state structures were confirmed by vibrational analysis. The BPW91 functional and the CCSD(T) method26 were also employed to complement the B3LYP results. The vibrational frequencies were calculated analytically, and zero-point energy is included in the calculation of binding and reaction energies. Previous investigations have shown that DFT calculated harmonic frequencies are usually a few percent higher than observed frequencies,8−10,27 and they provide useful predictions for infrared spectra of new molecules.
are labeled mp for metal phosphinidene (HPTiH2), and the strongest band at 1539.0 cm−1 is labeled i for the metal hydride phosphide insertion product (H2P-TiH) (see Table 1). Spectrum (b) shows that the mp bands almost double and the i band sharpens on annealing to 30 K, and scan (c) reveals a marked decrease in the mp bands upon λ > 290 nm irradiation, while the i feature increases about 10%. The final scan (d) recorded after annealing to 35 K shows a substantial increase in the mp bands. A weak band at 1435.5 cm−1 (not shown) is due to TiH2.28 The above 1610.1 and 1593.2 cm−1 bands are about the same relative intensity in solid argon, but the higher band comes on top of a water absorption, and its intensity is enhanced. The top set of spectra for a neon matrix sample shows absorptions shifted slightly to higher frequencies, 1641.5, 1625.5, 1544.3, and 637.5 cm−1, which is the expected behavior for a subject molecule interacting with a polarizable matrix where the unknown gas phase band would have a slightly higher frequency.29 The 1641.5 and 1625.5 cm−1 bands stand in a 5/6 relative intensity ratio, which is in excellent agreement with the 321/379 = 0.85 ratio for the computed infrared intensities of the symmetric and antisymmetric terminal Ti−H stretching modes for HPTiH2. Fortunately the neon matrix shifts these HPTiH2 absorptions higher and out of the water bending region of the infrared spectrum! Product absorptions are collected in Table 1. Reaction with deuterium enriched phosphines in excess argon gave stronger bands at 1174.7, 1156.8, 1111.3 cm−1 and a weaker band at 505.9 cm−1 for the two product species. The higher band H/D frequency ratios 1.371, 1.377, and 1.385 are appropriate for H and D vibrating against a mass like Ti. This ratio for TiH2/TiD2 is 1435.5/1041.1 = 1.379.28 The much lower 625.2/505.9 = 1.236 ratio indicates a different mode with less hydrogen motion. Following our earlier work with group 4 metal atom and methane reactions,7−9 we calculated the analogous bond insertion and double bonded product structures formed by αH-transfer to the metal using the B3LYP density functional, and these are illustrated in Figure 2 for Ti reacting with NH3, PH3, and AsH3. Notice that the phosphinidene and arsinidene structures are bent into highly agostic forms, whereas the imine structure is only slightly bent at the N center. This agrees with the structures of HNTiH2 and HPTiH2 reported earlier.10,12 The analogous arsinidenes (HAsMH2) have 4° smaller agostic angles than the phosphidenes, which is on the order of the 1.7° smaller valence angle for AsH3 as compared to PH3 (91.8° and 93.5°, respectively).30 An experiment with AsH3 and Ti gave similar, but weaker, absorptions at 1604.8, 1594.8, and 1538.9 cm−1. The deuterium counterpart was observed only for the stronger lower band at 1111.4 cm−1, which provided an H/D ratio of 1.385. The fact that the arsine product absorptions are close to the same frequencies as the phosphine products supports their assignments as our B3LYP calculations also give nearly the same product frequencies (listed in the SI). Zirconium. Infrared spectra for the Zr and PH3 reaction products in solid argon are illustrated in Figure 3. New absorptions on sample deposition at 1580.2 and 1554.6 cm−1 labeled mp and at 1520.2 cm−1 marked i increased 2-fold on annealing to 25 K and increased slightly upon exposure to the full light of our Hg arc lamp. Next, annealing to 31 K slightly increased the new bands, but the final 37 K annealing halved the i peak and increased the mp bands by 30%. No counterpart
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RESULTS AND DISCUSSION Infrared spectra and frequency, structure, and energy calculations will be considered for the group 4 metal atom reaction products with phosphine and arsine. Titanium. Figure 1 illustrates Ti and PH3 reaction products in solid argon and neon. Sharp argon matrix bands at 1610.1 and 1593.2 cm−1 and a very important weak peak at 625.2 cm−1
Figure 1. Infrared spectra of Ti and PH3 reaction products in solid argon (bottom) and neon (top). (a) Spectrum recorded after deposition of laser ablated Ti with 1% PH3 in argon for 50 min (w denotes water absorption), (b) after annealing to 30 K, (c) after irradiation (λ > 290 nm), and (d) after annealing to 35 K. (e) Spectrum recorded after deposition of laser ablated Ti with 1% PH3 in neon for 30 min and annealing to 8 K, (f) after annealing to 12 K, (g) after irradiation (λ > 480 nm), (h) after irradiation (λ > 220 nm), and (i) after annealing to 10 K. B
DOI: 10.1021/acs.inorgchem.6b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 1. Frequencies (cm−1) of Infrared Absorptions Assigned to Products in the Reactions of Group 4 Metal Atoms with Phosphine and Arsine in Solid Argon and Solid Neon argon
product HPTiH2(mp) DPTiD2(mp) H2P-TiH(i) D2P-TiD(i) HAsTiH2(ma) H2As−TiH(i) HPZrH2(mp) H2P−ZrH(i) DPZrD2(mp) D2P−ZrD (i) HAsZrH2(ma) H2As−ZrH(i) HPHfH2(mp) H2P−HfH(i) HAsHfH2(ma)
1610.1 1174.7 1539.0 1111.3 1604.8 1538.9 1580.2 1520.7 1131.7 1107.8 1580.0 1525.7 1648.7 not obs 1650.9
neon
1593.2 1156.8
625.5 505.9
1641.5
1625.5
637.5
1544.3 1594.8 1554.6
1596.7
1573.0
1676.4 1631.0
1666.0
1118.5 1555.0 1637.9
647.2
1640.7
Figure 3. Infrared spectra of Zr and PH3 reaction products in solid argon. (a) Spectrum recorded after deposition of laser ablated Zr with 2% PH3 in argon for 50 min, (b) after annealing to 25 K, (c) after irradiation (λ > 220 nm), (d) after annealing to 31 K, and (e) after annealing to 37 K. The w indicates water impurity.
Figure 2. Structures calculated using B3LYP and the aug-cc-pVTZ basis for the titanium insertion products with NH3 (ref 10a), PH3, and AsH3 and their H atom transfer imine, phosphinidene, and arsinedine products compared with CCSD(T) calculations for HPTiH2.
absorption was observed in the 600 cm−1 region. A deuterium enriched phosphine experiment gave counterpart bands at 1131.7, 1118.5 and 1107.8 cm−1.The spectrum using neon and lower laser energy gave weaker PH3 product bands at 1596.7 and 1573.0 cm−1. An analogous experiment with arsine produced a weaker set of product bands at 1580.0, 1555.0, and 1525.7 cm−1, which also increased on annealing. Figure 4 shows the computed structures for Zr, Hf, and Th phosphinidenes and arsinidenes. The agostic H−P−M angle increases just over 1° for each metal along the way from Ti (62.7°) to Th (66.8°). Hafnium. The top four spectra for the Hf and PH3 reaction during condensation in excess argon (Figure 5) show new mp bands at 1648.7 and 1637.9 cm−1, which increase on annealing
Figure 4. Phosphinidene (mp) and arsinidene (ma) structures calculated for the Zr, Hf, and Th reaction products with PH3 and AsH3.
to 20 and 30 K. No insertion product was observed. The bottom three spectra for the analogous reaction of Hf with AsH3 reveal new absorptions at 1650.9 and 1640.7 cm−1 labeled ma, which increase on annealing to 25 and 30 K. C
DOI: 10.1021/acs.inorgchem.6b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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H-PTiH2, H2P-TiH, H-AsTiH2, H2As-TiH. The four product absorptions in Figure 1 are grouped on the basis of different photolysis behavior. The mp bands are reduced more than 50% on UV irradiation, whereas the i band is affected much less in the argon matrix; hence, we have two different product species. The behavior of the pair of bands at 1610.1 and 1593.2 cm−1 suggests that they are due to the symmetric and antisymmetric stretching modes of a TiH2 subunit. Recall that these two bands are about 150 cm−1 higher than the antisymmetric stretching mode of isolated TiH2 at 1435.5 cm−1, but they are about 50 cm−1 lower than the 1656.7 cm−1 frequency for TiH4.28 The five highest computed frequencies and their intensities for the HNTiH2 product are 3570.2 cm−1 (100 km/mol, intensity), 1689.3 (254), 1654.4 (485), 1021.1 (203), and 630.4 (83) and for the HPTiH2 product are 2078.8 cm−1 (5 km/ mol), 1691.3 (321), 1671.1 (379), 847.3 (28), and 676.0 (103), and the bond stretching modes for the H2N−TiH product are 3580.6 cm−1 (21 km/mol), 3483.7 (20), and 1571.8 (432) and for H2P−TiH are 2353.1 cm−1 (26 km/mol), 2298.8 (46), and 1582.5 (429). These frequencies confirm identification of the product species and support their computed structures (complete sets of product frequencies are given in the Supporting Information). First, notice that the Ti−H2 stretching frequencies for HN TiH2 and HPTiH2 are nearly the same as are the two lower Ti−H stretching frequencies for H2N-TiH and H2P-TiH. Although the “free” N−H stretching frequencies for the two nitrogen bearing products are also nearly the same, the P−H stretching frequency for HPTiH2 is about 250 cm−1 lower than the average for H2P-TiH. This is a direct consequence of the strong agostic bonding in the phosphinidene as the agostic H interaction with the Ti center also reduces this P−H vibrational frequency. A similar effect is found in the arsinidene as the agostic H−As vibration in HAsTiH2 is 190 cm−1 lower than for the average “free” H−As stretching frequency in H2AsTiH. Unfortunately, these diagnostic P−H and As−H stretching modes are too weak to observe here. As can be seen from the models in Figure 2 computed with the B3LYP density functional, HNTiH2 has a Cs structure with two equal Ti−H bonds, whereas HPTiH2 and HAs TiH2 have highly agostic almost planar C1 structures. [The sum of the angles around Ti are 325.7° and 323.3°, respectively, using the BPW91 functional.] However, the angle sums are 360.0° for CCSD(T) calculations using two basis sets, so at the highest level of theory, HPTiH2 is a planar molecule. The natural TiE bond orders are 2.94, 1.96, and 1.95 for the E = N, P, and As products in B3LYP calculations. The natural bond orbitals compared for HPTiH2 and HAsTiH2 in Figure 7 are almost identical. The software plots the H···Ti agostic bonds for both species. The N contributes 42.0% and 49.1% s characters to the N−H and one of the Ti−N bonds, leading to an almost linear H-NiTi backbone. In contrast, P contributes 97.8% and 87.1% p character to the corresponding bonds in line with the highly bent Ti-P-H moiety, and As donates 98.2% and 88.9% p character. The metals contribute high d character to both sigma (87−90%) and pi (69−74%) EM bonds (see Table S2). Moreover, NBO analyses show strong electron donations for the P−H and As−H bonds to Ti, which contribute 228 and 273 kJ/mol of stabilization energy to the bonding. The strong electron donations are consistent with the highly agostic structures (∠HPTi and ∠HAsTi are 62.7° and 58.6°), while the electron donation energy in H2Ti-NH is only
Figure 5. Infrared spectra in the Hf−H stretching region for reaction products of laser ablated Hf atoms. (a) Spectrum after deposition of Hf with 2% AsH3 for 50 min, (b) after annealing to 25 K, (c) after annealing to 30 K, (d) spectrum after deposition of Hf with 2% PH3 for 50 min, (e) after annealing to 20 K, (f) after annealing to 30 K, and (g) after annealing to 38 K.
The reaction of Hf and PH3 during condensation at 4 K in excess neon, which is a slower process owing to the lower freezing point of neon (24 K), gave more intense product absorptions than the above argon matrix experiment with the higher freezing point of argon (84 K). Weak bands on deposition increased on annealing to 10 K (Figure 6, c),
Figure 6. Infrared spectra of Hf and PH3 reaction products in solid neon. (a) Spectrum recorded after deposition of laser ablated Hf with 1% PH3 in neon for 35 min, (b) after annealing to 8 K, (c) after annealing to 10 K, (d) after irradiation (λ > 220 nm), and (e) after annealing to 12 K. The p label indicates the precursor molecule PH3, and c denotes a band common to a number of different experiments.
increased slightly on λ > 220 nm irradiation (Figure 6, d), and increased further on final annealing to 12 K (Figure 6, e). The mp product gave split bands at 1676.4, 1672.6 and at 1666.0, 1663.0 cm−1 and an important weaker band at 647.2 cm−1, while the i product appeared at 1631.0 cm−1. The two Hf−H stretching modes for HPHfH2 blue-shifted by 27.7 and 28.1 cm−1, which is nearly the same as the two modes for HP TiH2, 31.4 and 32.3 cm−1, going from argon to neon matrix environments. Additional weaker bands at 1657 and 1648 cm−1 are probably due to the mp product perturbed by more phosphine. D
DOI: 10.1021/acs.inorgchem.6b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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In order to get some idea of the effect of the strong agostic interaction in the HPTiH2 and HAsTiH2 molecules, the H−P−Ti and the H−As−Ti angles were fixed at 170°, nearly the same as computed for nonagostic HNTiH2 (Figure 2), Cs symmetry was imposed, and their structures, frequencies, and energies were recalculated. These structures, presented in Figure S1, show changes associated with bonds to P and to As, but little difference for the Ti−H bonds. The optimized agostic forms are 36 kJ/mol more stable for the phosphinidene and 44 kJ/mol more stable for the arsinidene. These energies provide reliable estimates of the agostic bonding effects in these phosphidine and arsinidene species. Earlier calculations for CH2ZrH2 found the agostic ground-state structure to be 13 kJ/mol lower in energy than the fixed C2v strucuture.20 Frequencies are also listed in the SI for the fixed P−H and As−H bonded P species. Two other comparisons shed more light on the agostic bonding in these phosphinidene and arsinidene species (structures in Figures 2 and 4). Table 2 compares the computed agostic bond lengths with the sum of single bond covalent radii,32 which are essentially the same as the computed terminal Ti−H bond lengths. The Ti−H agostic bonds are 18% longer than a Ti−H single bond. It is useful to compare the bond length relationships in HPTiH2 with those in bridge bonded dialane, Al2H6.33,34 The B3LYP computed Al−H−Al angle, 97.6°, is considerably larger than the agostic angles for the two titanium dihydride species under examination here. However, the Al−H bridge bond is computed to be 174 pm, the terminal single bond 158 pm, and the bridge bond is 10% longer than the single bond. This compares favorably with the
Figure 7. P-Ti σ and π NBO’s from B3LYP/aug-cc-pVTZ calculation of HP-TiH2 and those of HAs-TiH2 with an isodensity of 0.04 e/Å. The occupation numbers are in the parentheses.
41 kJ/mol. These P and As complexes are far more agostic than the corresponding C products where the agostic angles (∠HCTi) are 91.3, 91.6, 86.7, and 85.9° for H2CTiH2, H2CTiHF, H2CTiHCl, and H2CTiHBr.7,31 The electron-rich third and fourth row elements produce strong agostic interactions between the E−H bond and Ti and lead to these unique, highly distorted agostic structures.
Table 2. Energies (kJ/mol) and Bond Lengths (pm) Calculated Using B3LYP for the Phosphide, Phosphinidene, Arsenide, and Arsinidene Reaction Products in (Singlet) and (Triplet) States moleculea
energyb
M−E distc
M + E radd
agostice
H + M radf
agostic/sum
HPTiH2(S) HP−TiH2(T) H2P−TiH(T) H2P−TiH(S) HAsTiH2(S) HAs−TiH2(T) H2As−TiH(T) H2As−TiH(S) HPZrH2(S) HP−ZrH2(T) H2P−ZrH(T) H2P−ZrH(S) HAsZrH2(S) HAs−ZrH2(T) H2As−ZrH(T) H2As−ZrH(S) HPHfH2(S) HP−HfH2(T) H2P−HfH(T) H2P−HfH(S) HAsHfH2(S) HAs−HfH2(T) H2As−HfH(T) H2As−HfH(S)
169 129 211 148 195 170 226 not cal 268 185 210 168 295 228 223 182 242 105 154 163 268 202 179 193
217
219
199/200g
168/172h
1.18
251
247
228
231
199
168/171
1.18
260
257
232
229
213/207
186/185
1.15
260
265
243
241
212/207
186/185
1.14
263
275
234
230
213/208g
184/185
1.16
258
263
243
242
212
184/184
1.15
257
273
a
Observed product in bold type. bEnergy lower than reagents, i.e., exothermic reactions. Calculation for H2As−TiH(S) converged to HAsTiH2. Metal−P or As bond distance, pm. dSum of single or double bond covalent radii, pm. eAgostic bond length, pm. fSum of single bond covalent radii, pm. gSecond number calculated with CCSD(T). hSecond number is calculated terminal Ti−H single bond length. c
E
DOI: 10.1021/acs.inorgchem.6b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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both are appropriately higher than the observed values, which does not provide a structure preference. However; the computed mode separations are 24.4 and 20.2 cm−1 for the nonagostic and agostic isomers, respectively, and the observed separation is 16.9 cm−1, which supports this identification of the agostic isomer observed here. A unique mode for the agostic isomer computed at 847.3 cm−1 is mostly vibration of the agostic hydrogen with some motion of the hydrides, which is confirmed by its high 1.388 H/D frequency ratio. Both isomers have computed modes in the 600 cm−1 region, the agostic at 676.0 cm−1 and the nonagostic at 635.8 cm−1. The former is mostly Ti−P−H and H−Ti−H bending motions, which is supported by its low 1.242 H/D ratio. The latter is mostly H−Ti−H bend and P−Ti stretch with a much lower 1.122 H/D ratio. Clearly, the 676.0 cm−1 band with a 1.242 H/D ratio computed for the agostic isomer agrees better with the properties of our observed 625.2 cm−1 band, namely, its diagnostic H/D ratio of 1.236, which provides additional vibrational mode evidence for the agostic structure. Calculations were done for HNTiH2, HPTiH2, and HAsTiH2 using the BPW91 pure density functional. The H− N−Ti angle was 2.4° larger, the H−P−Ti angle was 2.3° smaller, and the H−As−Ti angle was 2.3° smaller than for the B3LYP hybrid functional. Computed frequencies were slightly lower for the BPW91 functional (SI) as expected.27 H-PZrH2 and H2P-ZrH. The dominant features at 1580.2 and 1554.6 cm−1 are 77.4 and 87.0 cm−1 below the calculated Zr−H stretching frequencies, which is good agreement for the HPZrH2 molecule. The two hydride species ZrH2 at 1518.6 cm−1 and ZrH4 observed at 1623.6 cm−1 bracket the present observations.35 The intermediate bands at 1560 and 1570 cm−1 are probably due to the HPZrH2 product perturbed by more PH3, but we cannot be certain. Counterparts were observed for DPZrD2 at 1135.8 and 1118.5 cm−1, which give H/D ratios of 1.391 and 1.390. The slightly lower Zr−H stretching mode for the insertion product at 1520.2 cm−1 is predicted by calculation higher at 1599.9 cm−1, which is good agreement. In the Zr case, the higher metal oxidation state product is more stable due to stronger Zr−H bonds. Figure 9 illustrates this energy level diagram, which shows that the reaction proceeds smoothly through the Zr-PH3 complex to the lower energy phosphide and phosphinidene products. Table 2 provides the information to do the same analysis of the Zr−H agostic bonds as done above for the Ti−H bonds. The agostic bonds for Zr−H are 15% longer than their respective single bonds.20 Calculations at the DZP/CISD level by the Schaefer group33 gave slightly shorter bonds than our B3LYP calculations for Al2H6 and Ga2H6.34,36 Our bridged and terminal bonds for Ga2H6 were 177 and 156 pm, which reveals a 1.13 ratio and bridged bonds that are 13% longer than the terminal single bonds. Our data in Table 2 suggest that the agostic or bridged bonds are 15% longer than the terminal single bonds in the HEZrH2 molecules, which are in sufficiently good enough agreement with the values from doubly bridged digallane to certify that the agostic bonds in these Zr metal bearing molecules are also bridged bonds.
above relationships for HPTiH2, where the agostic or bridge bond is 18% longer than a single Ti−H bond. Therefore, we can conclude that the phosphinidene and arsinidene species observed here have sufficiently short agostic bond lengths to be considered as bridge bonds as well as agostic bonds. The Zr−H and Hf−H agostic bonds are relatively longer and weaker than the Ti−H counterparts. Table 2 also lists the B3LYP calculated reaction energies for the product species. In the Ti case, reaction to the phosphide product is more exothermic than to the phosphinidene (reactions 1 and 2). A potential energy diagram is given in Figure 8 for the ground-state Ti reaction with PH3 on the
Figure 8. Potential energy diagram for the reaction of ground-state Ti with PH3 calculated using the B3LYP density functional and the augcc-pVTZ basis set.
triplet potential energy surface, where the Ti + PH3 reagents are assigned the zero of energy. The Ti-PH3 complex is 66 kJ/ mol lower in energy, and there is a small 4.6 kJ/mol barrier to hold this intermediate. There is, however, a transition state HPTiH(H) 110 kJ/mol lower in energy between the phosphide and phosphinidene. As expected, singlet M-PH3 is higher in energy than triplet M-EH3: Ti-PH3, Zr-PH3, and Hf-PH3 are 27, 20, and 16 kJ/mol higher than the reactants, respectively. In the Ti reaction with ammonia, the Ti-NH3 complex was observed to increase on annealing and decrease on photolysis.10 However, the analogous Ti-PH3 complex is more reactive, and it is not observed here. Reactions 1 and 2 are sufficiently exothermic to activate the reactions to give both phosphide and phosphinidene products even though there is a transition state between them. Ti(3F2) + PH3 → H 2P−TiH Ti(3F2) + PH3 → HPTiH 2
ΔE = −211 kJ/mol ΔE = −169 kJ/mol
(1) (2)
Our B3LYP calculations predict the Ti−H stretching frequencies for HAsTiH2 to be 1.4 and 3.4 cm−1 higher than those for HPTiH2, but we instead observe them to be 5.3 and 4.1 cm−1 lower. Such small discrepancies are probably within the absolute accuracy of these frequency calculations. Next, we compare the computed Ti−H stretching frequencies for agostic HPTiH2 and the nonagostic geometric isomer (fixed H−PTi angle) (SI). The calculated Ti− H frequencies for each isomer are within 6 and 2 cm−1, and
Zr(3F2) + PH3 → H 2P−ZrH
Zr(3F2) + PH3 → HPZrH 2
ΔE = − 210 kJ/mol
(3)
ΔE = − 268 kJ/mol (4)
F
DOI: 10.1021/acs.inorgchem.6b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
the B3LYP density functional for the In2H6 molecule.38 These comparisons of relative agostic and terminal M−H bond lengths for group 4 metal dihydride bearing phosphinidene and arsinidene molecules and the relative bridged to terminal bond distances in the Al2H6, Ga2H6, and In2H6 family of dibridged molecules underscores the bridged nature of the agostic bonds in the former Ti, Zr, and Hf bearing molecules.
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CONCLUSIONS Reactions of Ti, Zr, and Hf atoms from laser ablation with PH3 or AsH3 in excess argon or neon produced phosphide or arsenide insertion products HM-PH2 or HM-AsH2 and phosphinidene or arsinidene molecules HPMH2 or HAs MH2. The latter are highly agostic with H−PTi, H−PZr, H−PHf angles of 62.7, 63.8 to 65.0° and HAsTi, HAs Zr, HAsHf angles from 58.6, 59.4 to 59.6°. The CCSD(T) calculations show that H−PTiH2 is a planar molecule. Both insertion and double bonded product absorptions increase on annealing the solid matrix samples, which suggests that these exothermic metal atom phosphine or arsine reactions are spontaneous. The phosphinidene HPTiH2(S) is 169 kJ/mol lower energy than the reactants, and the phosphide H2P-TiH(T) is 211 kJ/mol lower than Ti and PH3. Similar slightly lower energies were found for Ti and AsH3, and lower energies were observed for the analogous Zr and the Hf reaction products. The singlet HPMH2 and HAsMH2 species were more stable with the heavier metals owing to stronger heavier metal M−H bonds relative to weaker P−H and As−H bonds. Comparisons are made with the agostic M−H bond lengths, the terminal single M−H lengths, and the sum of covalent single bond radii.32 The bridged and terminal M−H bond lengths in Al2H6, Ga2H6, and In2H6 are in 1.10, 1.13, and 1.13 ratios, which are nearly the same as the relative agostic and terminal M−H bond length ratios of 1.18, 1.15, and 1.16 found for the present HEMH2 molecules. This comparison shows clearly that the strong agostic bonds in these phosphinidene and arsinidene molecules can also be viewed as bridged bonds.
Figure 9. Potential energy diagram for the reaction of ground-state Zr with PH3 calculated using the B3LYP density functional and the augcc-pVTZ basis set for H and P and aug-cc-pVTZ-pp for Zr.
H-PHfH2, H2P-HfH, H-AsHfH2, H2As-HfH. Figure 5 compares spectra in the Hf−H stretching region for the major PH3 and AsH3 reaction products, which were observed at 1648.7, 1637.9 cm−1 for PH3 and at 1650.9, 1640.7 cm−1 for AsH3 (labeled mp and ma). These product frequencies were calculated for H−PHfH2 and H−AsHfH2 at 1710.1, 1682.9 cm−1 and 1711.4, 1683.0 cm−1. First, these bands are predicted about 61 and 43 cm−1 higher than observed, which is again very good agreement for these heavy metal molecules.20,21,27 These new Hf−H stretching modes fall between antisymmetric stretching modes for HfH2 at 1622.4 cm−1 and HfH4 at 1678.4 cm−1 reported from earlier argon matrix experiments.35 Notice that the M−H frequencies reported here for HPTiH2 decrease 30 and 40 cm−1 for HPZrH2 and then increase 68 and 83 cm−1 for HPHfH2. This increase from Zr to Hf was observed before with the hydrides and explained by relativistic contraction on going to the third row transition metal element.35,37 The H−P−Hf mode involving the agostic hydrogen was observed in solid neon at 647.2 cm−1, just 10 cm−1 higher than the Ti counterpart. Finally, our calculations predict 1.3 and 0.1 cm−1 higher Hf−H stretching frequencies for the arsinidene than the phosphinidene, and we observed them to be 2.2 and 2.8 cm−1 higher, all well within the absolute accuracy of these DFT calculations. The structures in Figure 4 show that HPZrH2 and HPHfH2 are slightly more deflected from planar than HPTiH2 [sum of angles 317.4° and 317.8°]. Finally, the Hf reactions are less energetic than those for Zr, but the phosphinidene is still more stable than the phosphide (reactions 5 and 6).
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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.6b01276. Calculated frequencies, structures with fixed HPTi angle, and NBO tables (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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(6)
ACKNOWLEDGMENTS We gratefully acknowledge financial support from TIAA for L.A., and support from the Korea Research Foundation (KRF) grant funded by the Korean Government (NRF2013R1A1A2060088) and KISTI supercomputing center.
Table 2 also provides information on the agostic and terminal Hf−H single bond lengths for the HPHfH2 and HAsHfH2 molecules, and their 1.16 ratio is near the 1.13 ratio for bridged to terminal In−H bond lengths calculated by
(1) Rostas, J.; Cossart, D.; Bastien, J. R. Can. J. Phys. 1974, 52, 1274− 1287.
Hf(3F2) + PH3 → H 2P−HfH Hf(3F2) + PH3 → HPHfH 2
ΔE = −154 kJ/mol
(5)
ΔE = −242 kJ/mol
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REFERENCES
DOI: 10.1021/acs.inorgchem.6b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (2) Ram, R. S.; Bernath, P. F. J. Mol. Spectrosc. 1987, 122, 275−281. (3) Goto, M.; Saito, S. Chem. Phys. Lett. 1993, 211, 443−448. (4) Larzilliere, M.; Damany, N.; Lan Thanh, M. Can. J. Phys. 1979, 57, 539−548. (5) Larzilliere, M.; Jacox, M. E. J. Mol. Spectrosc. 1980, 79, 132−150. (6) Withnall, R.; Andrews, L. J. Phys. Chem. 1987, 91, 784−797. (7) Andrews, L.; Cho, H.-G; Wang, X. Inorg. Chem. 2005, 44, 4834− 4842 (Ti + CH4). (8) Andrews, L.; Cho, H.-G; Wang, X. Angew. Chem., Int. Ed. 2005, 44, 113−116 (Zr + CH4). (9) Cho, H.-G; Wang, X.; Andrews, L. Organometallics 2005, 24, 2854−2861 (Hf + CH4). (10) (a) Chen, M.; Lu, H.; Dong, J.; Miao, L.; Zhou, M.-F. J. Phys. Chem. A 2002, 106, 11456−11464. (Sc, Ti, V + NH3) (b) Zhou, M.F.; Chen, M.; Zhang, L.; Lu, H. J. Phys. Chem. A 2002, 106, 9017− 9023 (Zr, Hf + NH3). (11) Wang, X. F.; Andrews, L.; Marsden, C. J. Chem.Eur. J. 2007, 13, 5601−5606. (12) Chung, G.; Gordon, M. S. Organometallics 2003, 22, 42−46. (13) Ehlers, A. W.; Baerends, E. J.; Lammertsma, K. J. Am. Chem. Soc. 2002, 124, 2831−2838. (14) Aktas, H.; Slootweg, J. C.; Lammertsma, K. Angew. Chem., Int. Ed. 2010, 49, 2102−2113. (15) Wang, Y.; Xie, Y.; Abraham, M. Y.; Gilliard, R. J., Jr.; Wei, P.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. Organometallics 2010, 29, 4778−4780. (16) Waterman, R.; Tilley, T. D. Chem. Sci. 2011, 2, 1320−1325. (17) Bonanno, J. B.; Wolczanski, P. T.; Lobkovsky, E. B. J. Am. Chem. Soc. 1994, 116, 11159−11160. (18) Pugh, T.; Kerridge, A.; Layfield, R. A. Angew. Chem., Int. Ed. 2015, 54, 4255−4258. (19) Seidl, M.; Kuntz, M.; Bodensteiner, M.; Timoshkin, A. Y.; Scheer, M. Angew. Chem., Int. Ed. 2015, 54, 2771−2775. (20) Andrews, L.; Cho, H.-G Organometallics 2006, 25, 4040−4053 (review). (21) Andrews, L. Chem. Soc. Rev. 2004, 33, 123−132 (transition metal hydrides). (22) 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, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (23) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C.; Yang, Y.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (24) aug-cc-pVTZ basis sets in Gaussian 09. (a) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007−1023 (H). (b) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796−6806 (B-Ne). (c) Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, 1358− 1371 (Al-Ar). (d) Balabanov, N. B.; Peterson, K. A. J. Chem. Phys. 2005, 123, 064107 (Sc-Zn). (e) Peterson, K. A.; Figgen, D.; Dolg, M.; Stoll, H. J. Chem. Phys. 2007, 126, 124101 (Y-Pd). (f) Figgen, D.; Peterson, K. A.; Dolg, M.; Stoll, H. J. Chem. Phys. 2009, 130, 164108 (Hf-Pt). (25) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265−3269. (26) (a) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (b) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density
Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum: New York, 1998. (c) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968−5975. (27) (a) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502− 16513. (b) Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 2937−2941. (28) Jacox, M. E. Chem. Soc. Rev. 2002, 31, 108−115. (29) Chertihin, G. V.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 8322−8327. (30) Nielsen, H. H. J. Chem. Phys. 1952, 20, 759 The smaller angles indicate that the electron lone pair is more diffuse than in ammonia, and the repulsion between the hydrogen atoms is weaker due to the longer P−H and As−H bonds.. (31) Cho, H.-G.; Andrews, H.-G. Inorg. Chem. 2005, 44, 979−988. (32) Pyykkö, P.; Atsumi, M. Chem.Eur. J. 2009, 15, 12770−12777. (33) Duke, B. J.; Liang, C.; Schaefer, H. F., III J. Am. Chem. Soc. 1991, 113, 2884−2890 (Al2H6 and Ga2H6). (34) Andrews, L.; Wang, X.-F. Science 2003, 299, 2049−2052 (Al2H6). (35) Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1995, 99, 15004− 15010. (36) Wang, X.-F.; Andrews, L. J. Phys. Chem. A 2003, 107, 11371− 11379 (Ga2H6). (37) Pyykkö, P. Chem. Rev. 1988, 88, 563−594. (38) Wang, X.-F.; Andrews, L. J. Phys. Chem. A 2004, 108, 4440− 4448 (In2H6).
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DOI: 10.1021/acs.inorgchem.6b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
