Fluoroborylene Complexes FBMF2 (M = Sc, Y, La, Ce): Matrix Infrared

Jan 15, 2019 - Laser-ablated group 3 transition metal and cerium atom reactions with boron trifluoride were explored in excess solid neon at 4 K throu...
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Fluoroborylene Complexes FBMF2 (M = Sc, Y, La, Ce): Matrix Infrared Spectra and Quantum Chemical Calculations Bing Xu,*,† Li Li,† Zhen Pu,‡ Wenjie Yu,† Wenjing Li,† and Xuefeng Wang*,† †

School of Chemical Science and Engineering, Shanghai Key Lab of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092, People’s Republic of China ‡ China Academy of Engineering and Physics, Mianshan Road, Mianyang 621907, People’s Republic of China

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ABSTRACT: Laser-ablated group 3 transition metal and cerium atom reactions with boron trifluoride were explored in excess solid neon at 4 K through matrix isolation infrared spectroscopy and quantum chemical calculations. The fluoroborylene complexes FBMF2 (M = Sc, Y, La, Ce) were trapped in inert gas and identified by the isotopic substitutions. The observed frequencies of FBMF2 were reproduced by DFT, NEVPT2, and CASSCF calculations. From Sc to La, the observed F−11B stretching mode has been observed at 1391.9 cm−1 (Sc), 1370.8 cm−1 (Y), and 1337.1 cm−1(La); however, for Ce this mode shifts up to 1340.8 cm−1, which is due to relativistic effects. The electron localization function (ELF) analysis and the theory of atoms in molecules (AIM) were applied to investigate the character of the B−M bond in FBMF2 molecules, which favors bond order 1.5.

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width) was focused onto a rotating freshly cleaned metal target (Sc, Y, La, and Ce), which gave a bright plume reacting with BF3 and deposited uniformly onto a 4 K CsI window cooled by a closed-cycle helium refrigerator (Sumitomo Heavy Industries Model RDK 205D). The samples of 11BF3 and 10BF3 were purchased from Jinglin Chemical Industry Limited Liability Company (Shanghai, China, chemical purity ≥99.99%). The laser-ablated metal atoms (Sc, Y, La, and Ce) react with BF3 in excess neon for 50 min at a rate of 2−4 mmol/h. Infrared spectra were recorded on a Bruker 80V spectrometer at 0.5 cm−1 resolution between 4000 and 400 cm−1 using a HgCdTe range B detector. Then samples were annealed at different temperatures and later irradiated by a mercury arc lamp (Philips, 175 W), and more spectra were recorded. To provide detailed information, theoretical calculations were performed to predict the structures and vibrational frequencies of reaction products by using both DFT and ab initio methods. DFT computation, via the Gaussian 09 program,9 was performed mainly with the B3LYP hybrid functional.10,11 The 6-311++G(3df,3pd) basis set was applied for boron and fluorine,12 while the SDD pseudopotential basis set was used for scandium, yttrium, lanthanum, and cerium atoms.13,14 In addition to B3LYP, other functionals were also used for geometry optimizations. The orbital composition, as well as Wiberg bond index analyses,15 was calculated by a natural bond orbital (NBO) population analysis.9,16 Moreover, the electron localization function (ELF)17 and the theory of atoms in molecules (AIM)18 analysis, performed by the Multiwfn code,19 were applied to investigate the bonding characters. In the case of ab initio calculations, high-level multireference methods were used to obtain accurate information on the electronic structures of FBMF2 compounds by the ORCA 4.0.1 program.20,21 For Sc, Y, and La, the active space of the CASSCF22 is three active electrons in nine active orbitals: i.e., CAS(3e,9o). For Ce, the active

INTRODUCTION The fixation of CO by a borylene-like allenic dicoordinate (RBL) species was reported in 2014, and strong p backbonding from boron to CO in a number of borylene complexes was observed.1 The suitability of borylenes as candidates for N2 binding was further underlined by a recent report of N2 binding with the unstabilized borylene “PhB”: under matrix conditions.2 In borylene complexes both Lewis acidity and basicity are manifested simultaneously, which can be modified by an active substitutional group to alter the reactivity.3 Fluoroborylene (FB:), which is an isoelectronic relative of CO and N2, superbly exemplifies the substituted borylene with numerous bonding and chemical trends.4 The simplest fluoroborylene, the diatomic BF molecule, was prepared at high temperatures by Timms in the reaction of BF3 with solid boron in 1967.5 There were few fluoroborylene molecules known until recently the borylene complexes FB−MF2 (M = Ti, Zr, Hf, Th) were isolated in a low-temperature matrix.6,7 In this paper, we report the reactions of laser-ablated Sc, Y, La, and Ce atoms with BF3 molecules in excess solid noble gases. The fluoroborylene complexes FBMF2 (M = Sc, Y, La, Ce) were identified by boron isotopic substitution and theoretical frequency calculations. Our theoretical investigations show that fluoroborylene BF is stabilized by one σ bond and one one-electron π bond between the B atom and metal atom.

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EXPERIMENTAL AND COMPUTATIONAL METHODS

The experimental equipment and procedure for studying the reaction of the laser-ablated metal atom with BF3 during codeposition in excess neon at 4 K have been reported previously.8 In short, the Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse © XXXX American Chemical Society

Received: October 2, 2018

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DOI: 10.1021/acs.inorgchem.8b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Infrared spectra of the laser-ablated Sc atoms reactions with BF3 in excess solid neon: (a) codeposition of Sc + 0.3% 11BF3 for 50 min; (b) after annealing to 8 K; (c) after λ >400 nm irradiation for 6 min; (d) after annealing to 12 K; (e) codeposition of Sc + 0.25% 10BF3 for 50 min; (f) after annealing to 8 K; (g) after λ >400 nm irradiation for 6 min; (h) after annealing to 12 K.

Table 1. Observed and Calculated Fundamental Frequencies of FBScF2 Isotopomers in the Ground 2A′ Statea F11BScF2 approx description

obsdb

B−F str Sc−F antisym str Sc−F sym str B−Sc str ScBF def ScBF def ScF2 bend FBScF def BScF2 def

1391.9 693.4 629.7

calcd (int) B3LYP 1425.4 694.7 629.8 344.2 279.9 254.4 148.0 85.6 71.2

(414.3) (239.4) (179.5) (3.5) (0.2) (12.7) (10.6) (2.0) (50.4)

F10BScF2

calcd (int) NEVPT2(3e,6o)/ Def2-TZVP 1439.7 765.6 637.4 331.8 286.6 263.4 129.0 123.4 118.0

(518.6) (264.1) (235.5) (0.9) (11.7) (1.39) (20.9) (69.1) (7.95)

calcd (int) CAS(3e,9o)/ Def2-QZVP 1485.6 (488.8) 724.4 (245.7) 667.9 (211.5) 465.8 (0.6) 382.41 (13.3) 370.3 (0.2) 227.7 (68.9) 177.6 (13.5) 145.6 (6.6)

obsdb 1437.6 693.5 629.7

calcd (int) B3LYP 1472.6 694.8 629.8 346.9 291.0 263.9 148.4 85.6 71.2

(441.6) (239.4) (179.6) (3.6) (0.2) (12.7) (10.6) (2.0) (50.6)

The vibrational frequencies (cm−1) and intensities (km/mol, in parentheses) are calculated using B3LYP/6-311++g(3df,3pd)/SDD, NEVPT2(3e,6o)/Def2-TZVP, and CAS(3e,9o)/Def2-QZVP. bObserved in a neon matrix. a

and Ce atoms with 11BF3 or 10BF3 in a neon matrix. In addition the calculated frequencies with DFT, NEVPT2, and CASSCF are given in the same tables for comparison. These product absorptions responded to the stepwise annealing and irradiation and will be discussed below. Some precursor fragments, such as BF2, BF, and HBF2, have been reported previously.6,7 In addition, the absorptions due to impurities such as H2O, CO2, and CO in the experiments can be reduced as much as possible. FBScF2. The infrared spectra for the reactions of laserablated Sc atoms codeposited with 11BF3 in excess neon and their following behavior on annealing and broad photolysis are shown in Figure 1. New bands appeared at 1391.9, 693.4, and 629.7 cm−1 after codeposition, having no significant change on annealing to 8 K, but they decreased by nearly half after λ >400 nm irradiation and increased slightly on annealing to 12 K later. The experiment was repeated under the isotopic gas

space is CAS(4e,16o). The effect of dynamic correlation was taken into account by NEVPT224−26 on top of wave functions at the CASSCF level to get more accurate energies. However, due to the large computation cost, smaller active spaces, which are CAS(3e,6o) for Sc, Y, and La and CAS(4e 7o) for Ce, were used for NEVPT224−26 calculations. Def2-QZVP was used for CASSCF, and NEVPT2 calculations were performed with Def2-TZVP. In our molecules, the contributions from the main configuration are 95.04%, 95.08% and 94.82% for FBScF2, FBYF2, and FBLaF2, respectively, at CAS(3e,9o)/Def2-TZVP and 94.12% for FBCeF2 at CAS(4e,16o)/Def2-TZVP. Thus, according to ab initio results, the multireference character of our molecules should be small since the contribution from the main configuration approaches 100%.

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RESULTS AND DISCUSSION The typical infrared spectra in the selected regions are illustrated in Figures 1−6, and the absorption bands are given in Tables 1−4 for the reaction products of laser-ablated Sc, Y, La, B

DOI: 10.1021/acs.inorgchem.8b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Infrared spectra of the laser-ablated Y atom reactions with BF3 in excess solid neon: (a) codeposition of Y + 0.3% 11BF3 for 50 min; (b) after annealing to 8 K; (c) after λ >300 nm irradiation for 6 min; (d) after annealing to 12 K; (e) codeposition of Y + 0.3% 10 BF3 for 50 min; (f) after annealing to 8 K; (g) after λ >300 nm irradiation for 6 min; (h) after annealing to 12 K.

Figure 3. Infrared spectra of the laser-ablated La atom reactions with BF3 in excess solid neon: (a) codeposition of La + 0.3% 11BF3 for 50 min; (b) after annealing to 8 K; (c) after λ 365 nm irradiation for 6 min; (d) after annealing to 12 K; (e) codeposition of La + 0.3% 10 BF3 for 50 min; (f) after annealing to 8 K; (g) after λ 365 nm irradiation for 6 min; (h) after annealing to 12 K.

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BF3 under the same conditions, and the absorptions shifted to 1437.6, 693.5, and 629.7 cm−1, respectively, which had photochemical behavior similar to that for the bands for 11BF3. The 1391.9 cm−1 band produced in the reaction of Sc with 11 BF3 is close to the B−F stretching vibration modes observed at 1404, 1373, and 1378 cm−1 for FB−MF2 (M = Ti, Zr, Hf), respectively.6 This band shows an 10B/11B isotopic frequency ratio of 1.0328, which is in good agreement with the boron isotopic frequency ratio for the characteristic B−F stretching vibration in FBMF2 (M = Ti, Zr, Hf) complexes6,7 and BX (X = F, Cl, Br, I).27,28 The observed bands at 693.4 and 629.7 cm−1 hardly shift with boron isotopic substitution (bands at 693.5 and 629.7 cm−1 in Figure 1, top), but two bands are close to the F−Sc−F vibrational modes for ScF2 (697.5 and 669.9 cm−1 in Ar)29 and the same modes in H2CScF2 (690.5 and 633.1 cm−1 in Ar)30 and in OScF2 (689.0 and 665.3 cm−1 in Ar),31 suggesting that the two bands are possibly due to the antisymmetric and symmetric stretching vibrations of F−Sc−F modes, respectively.

The DFT computations predicted that the FBScF2 molecule has Cs symmetry with 2A′ ground electronic state, and the calculated frequencies are given in Table 1. With B3LYP the calculated frequencies are all slightly higher than the observed values, in which the F−11B(10B) stretching mode and the Sc−F antisymmetric and symmetric frequencies are only overestimated by 33.5 (35.0), 1.3 (1.3), and 0.1 (0.1) cm−1, respectively. Furthermore, the calculated 10B/11B isotopic frequency ratio 1.0331 agrees very well with the experimental value. Our NEVPT2 frequency calculation also gave very good predictions, and the F−11B stretching mode and the Sc−F unti-symmetric and symmetric frequencies are 47.8, 72.2, and 7.7 cm−1 higher than the observed values. Notice that the deviations are slightly more with CASSCF calculated frequencies (Table 1). FBYF2. Reactions of the laser-ablated Y atoms with 11BF3 and 10BF3 in solid neon were shown in Figure 2. New absorption bands at 1370.8, 579.6, and 569.9 cm−1 were given after

Table 2. Observed and Calculated Fundamental Frequencies of FBYF2 Isotopomers in the Ground 2A′ Statea F11BYF2 approx description B−F str Y−F antisym str Y−F sym str B−Y str YBF def YBF def YF2 bend FBYF def BYF2 def

obsdb 1370.8 579.6 569.9

F10BYF2

calcd (int) B3LYP

calcd (int) NEVPT2(3e,6o)/ Def2-TZVP

calcd (int) CAS(3e,9o)/ Def2-QZVP

1396.9(336.5) 586.5(203.3) 565.3(123.8) 306.7(17.4) 261.2(0.6) 240.8(8.0) 138.1(12.0) 83.7(3.1) 40.2(46.7)

1410.0(387.4) 629.3(217.9) 583.5(146.6) 343.4(12.8) 348.4(0.8) 313.3(11.9) 150.2(11.6) 80.1(48.4) 13.2(32.4)

1446.0(415.8) 592.2(212.1) 583.2(139.7) 305.8(28.7) 244.2(2.1) 208.0(2.5) 148.7(18.2) 96.4(53.8) 87.9(5.6)

obsdb 1415.8 579.8 570.1

calcd (int) B3LYP 1443.3(355.2) 586.6(203.5) 565.3(123.8) 309.8(17.7) 271.7(0.7) 250.1(8.4) 138.3(12.0) 83.7(3.1) 40.2(46.7)

The vibrational frequencies (cm−1) and intensities (km/mol, in parentheses) are calculated using B3LYP/6-311++g(3df,3pd)/SDD, NEVPT2(3e,6o)/Def2-TZVP, and CAS(3e,9o)/Def2-QZVP. bObserved in a neon matrix. a

C

DOI: 10.1021/acs.inorgchem.8b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 3. Observed and Calculated Fundamental Frequencies of FBLaF2 Isotopomers in the Ground 2A′ Statea F11BLaF2 approx description B−F str La−F sym str La−F antisym str B−La str LaBF def LaBF def LaF2 bend FBLaF def BLaF2 def

obsdb 1337.1, 1331.9 (site) 513.0 499.9

F10BLaF2

calcd (int) B3LYP

calcd (int) NEVPT2(3e,6o)/ Def2-TZVP

calcd (int) CAS(3e,9o)/ Def2-QZVP

1357.3 (361.9)

1388.8 (526.9)

1419.5 (505.4)

502.0 (138.7) 492.4 (236.7)

594.5 (230.1) 501.8 (133.9)

516.8 (150.2) 505.1 (232.7)

259.1 227.4 226.8 119.9 71.2 48.7

319.6 257.0 200.0 180.1 107.9 23.1

268.0 232.7 197.2 140.5 85.0 74.5

(20.8) (18.6) (3.2) (10.0) (3.3) (36.2)

(27.5) (12.0) (8.3) (11.4) (9.2) (45.4)

(27.4) (7.7) (4.6) (13.1) (46.5) (4.5)

obsdb 1380.3, 1374.8 (site) 513.1 500.1

calcd (int) B3LYP 1401.7 (380.9) 502.0 (138.5) 492.5 (237.2) 262.2 236.2 235.9 120.1 71.2 48.7

(21.3) (20.0) (3.4) (10.1) (3.3) (36.2)

a The vibrational frequencies (cm−1) and intensities (km/mol, in parentheses) are calculated using B3LYP/6-311++g(3df,3pd)/SDD, NEVPT2(3e,6o)/Def2-TZVP, and CAS(3e,9o)/Def2-QZVP. bObserved in a neon matrix.

557.9 cm−1 in Ar)32 and YF2 (571.4 cm−1 in Ar),29 suggesting that the two bands are due to the Y−F antisymmetric and symmetric stretching vibration modes, respectively. The assignment of the FBYF2 molecule is further confirmed by our frequency calculations. The B3LYP/NEVPT2/CASSCF calculations predicted the FBYF2 molecule to have Cs symmetry with 2A′ ground electronic state. As given in Table 2, our B3LYP calculation manifested that the 11B (10B)−F and Y−F stretching modes reproduced the observed values very well, only overestimated by 26.1 (27.5) and 6.9 (6.8) cm−1, respectively. The calculated 10B/11B isotopic frequency ratio of 1.0332 is in very good agreement. The NEVPT2 values are slightly higher than the observed values by 39.2, 49.7, and 13.6 cm−1 for 11B−F and Y−F stretching modes, respectively, and are in good agreement. The CASSCF values are higher than the observed values by 75.2, 12.6, and 13.3 cm−1 for the three stretching modes. FBLaF2. The spectra of reactions for laser-ablated La atoms and BF3 in solid neon are illustrated in Figure 3. New absorption bands appeared at 1337.1 (site at 1331.9), 513.0, and 499.9 cm−1 after codeposition in reaction with 11BF3, which increased slightly on annealing to 8 K but decreased about 50% after λ 365 nm irradiation and increased again by about 20% on finally annealing to 12 K. For the complementary experiment of La with 10BF3 (Figure 3, top), these observed bands shift to 1380.3 (site at 1374.8), 513.1, and 500.1 cm−1, which had the same behaviors as their counterparts in 11BF3. The observed band at 1337.1 cm−1 shifted to 1380.3 cm−1 with 10BF3 isotopic substitution (Figure 3), giving a 10B/11B isotopic frequency ratio of 1.0323, which fits the characteristic B−F vibration mode.7 The observed bands at 513.0 and 499.9 cm−1 have nearly no obvious shifts on isotopic substitution (bands at 513.1 and 500.1 cm−1 in Figure 3, top), which are close to the F−La−F vibrational modes in HC(F)LaF2 (521.0 and 499.9 cm−1 in Ne)33 and in OLaF2 (502.3 and 478.3 cm−1 in Ar),34 suggesting that the two bands are possibly due to the symmetric and the antisymmetric stretching vibrations of the F−La−F mode, respectively. The B3LYP/NEVPT2/CASSCF calculations predicted the FBLaF2 molecule to have Cs symmetry with 2A′ ground electronic state. The experimental results for FBLaF2 are modeled very well by B3LYP frequency calculations. As given in Table 3, the 11B (10B)−F stretching mode is overestimated by 20.2 (21.4) cm−1 at the B3LYP level and the calculated 10B/11B isotopic frequency ratio of 1.0327 fits the observed value very well.

Figure 4. Infrared spectra of the laser-ablated La atom reactions with BF3 in excess solid neon: (a) codeposition of Ce + 0.5% 11BF3 for 50 min; (b) after annealing to 12 K; (c) after λ >220 nm irradiation for 6 min; (d) after annealing to 12 K; (e) codeposition of Ce + 0.5% 10 BF3 for 50 min; (f) after annealing to 12 K; (g) after λ >220 nm irradiation for 6 min; (h) after annealing to 12 K.

codeposition in reaction with 11BF3, which nearly had no change upon annealing to 8 K but decreased about 50% after λ >300 nm irradiation. For the reaction of Y with 10BF3 (Figure 2, top), a new group of bands were observed at 1415.8, 579.8, and 570.1 cm−1, which had behavior similar to that of their counterparts for 11BF3. This group of bands are assigned to the FBYF2 molecule. First, the 1370.8 cm−1 band is very close to the values 1373.0 and 1378.0 cm−1 of the B−F stretching mode in FBZrF2 and FBHfF2, respectively.6 The 1370.8 cm−1 band shifts to 1415.8 cm−1 in the 10BF3 isotopic substitution experiment, exhibiting a boron isotopic frequency ratio of 1.0328 that is close to the previously reported boron isotopic frequency ratio (1.0329) of the FBThF2 molecule.7 The observed bands at 579.6 and 569.9 cm−1 have no distinct shifts on isotopic substitution (bands at 579.8 and 570.1 cm−1 in Figure 2, top) and are close to the F−Y−F vibrational modes in OYF2 (560.7 and D

DOI: 10.1021/acs.inorgchem.8b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Infrared spectra of the laser-ablated metal atoms (Sc, Y, La, Ce) reactions with BF3 in excess solid neon after annealing to 12 K: (a) Ce + 0.5% 11BF3; (b) La + 0.3% 11BF3; (c) Y + 0.3% 11BF3; (d) Sc + 0.3% 11BF3.

Table 4. Calculated and Observed Fundamental Frequencies of FBCeF2 Isotopomers in the Ground 3A″ Statea F11BCeF2 approx description B−F str Ce−F sym str Ce−F antisym str Ce−B str CeBF def CeBF def CeF2 bend FBCeF def BCeF2 def

obsdb 1340.8 522.0 503.4

calcd (int) B3LYP 1360.4 503.6 495.6 258.8 228.9 226.5 122.7 72.1 67.9

(367.0) (159.8) (222.2) (14.7) (3.0) (19.3) (11.6) (3.3) (37.6)

calcd (int) NEVPT2(4e,7o)/ Def2-TZVP 1375.2 560.8 519.8 258.4 255.6 159.5 102.3 63.6 51.4

F10BCeF2 calcd (int) calcd (int) CAS(4e,16o)/ Def2-QZVP

(472.0) (234.2) (189.7) (4.0) (1.7) (33.1) (3.4) (12.3) (36.7)

1449.9 525.4 509.9 415.5 298.2 259.3 134.8 91.5 73.7

(551.3) (155.9) (246.4) (0.6) (0.5) (8.8) (16.9) (44.7) (4.7)

obsdb 1386.3 522.0 503.4

calcd (int) B3LYP 1404.8 503.5 495.7 261.9 237.8 235.3 122.9 72.0 67.9

(387.4) (159.7) (222.7) (15.0) (3.2) (20.7) (11.7) (3.3) (37.6)

a The vibrational frequencies (cm−1) and intensities (km/mol, in parentheses) are calculated using B3LYP/6-311++g(3df,3pd)/SDD, NEVPT2(4e,7o)/Def2-TZVP, and CAS(4e,16o)/Def2-QZVP. bObserved in a neon matrix.

Table 5. Relative Energies (kcal mol−1) of F2BMF and FBMF2 (M = Sc, Y, La, Ce) in Comparison to Reagents and B−M Bond Dissociation Energy (BDE (kcal mol−1))

However, the symmetric and antisymmetric stretching vibrations of the F−La−F mode are all slightly underestimated by 11.0 and 7.5 cm−1 for F11BCeF2 and 11.1 and 7.6 cm−1 for F10BCeF2, respectively. Note that the NEVPT2 and CAS results all overestimate these three stretching modes (Table 3). FBCeF2. The product absorptions from the reaction of laserablated cerium atoms with 11BF3 in a neon matrix appeared at 1340.8, 522.0, and 503.4 cm−1 after codeposition. These bands increased slowly on annealing to 12 K and decreased after >220 nm irradiation but increased upon annealing to 12 K again. The experiment was repeated under the isotopic gas 10 BF3 in excess solid neon, and the absorption bands appeared at 1386.3, 522.0, and 503.4 cm−1 increased on annealing to 12 K but had no response to broad-band irradiation. The absorption band appearing at 1340.8 cm−1 in the reaction of Ce with 11BF3 is near the B−F stretching vibration of FBMF2 (M = Ti, Zr, Hf, Th),6,7 indicating that this band might be due to a terminal B−F stretching vibration. The band shifts to 1386.3 cm−1 upon 10B substitution (Figure 4, top), giving a 10B/11B isotopic frequency ratio of 1.0339, which is in line with previous studies,7,27,28 indicating the assignment of this diagnostic vibrational mode. In Figure 4, the experimentally observed bands at 522.0 and 503.4 cm−1 have nearly no shifts with boron isotopic substitution, which are close to the F−Ce−F vibrational modes in HC(F)CeF2 (492.1 cm−1)33

M + BF3 F2BMF FBMF2 BDE of BM

Sc

Y

La

Ce

0 −28.7 −41.2 27.0

0 −37.8 −46.6 27.9

0 −39.3 −58.4 29.6

0 −41.2 −52.3 31.5

and in OCeF2 (487.5 cm−1, 516.6 cm−1),35 suggesting that the two bands are possibly due to the symmetric and antisymmetric stretching vibrations of F−Ce−F modes, respectively. This group of bands are assigned to the FBCeF2 molecule. Similar frequency calculations were done for FBCeF2, and the results are given in Table 4. The observed values are reproduced very well by B3LYP, with which the 11B (10B)−F stretching mode is overestimated by 19.6 (18.5) cm−1 and the calculated 10B/11B isotopic ratio of 1.0326 matches the experimental value very well. The calculated Ce−F stretching modes are underestimated by 18.4 and 7.8 cm−1 for F11BCeF2 and 18.5 and 7.7 cm−1 for F10BCeF2 cm−1, respectively. With NEVPT2 the calculated frequencies are overestimated by 34.4, 38.8, and 16.4 cm−1 for the three stretching modes, which are in good agreement. Note with CAS calculations that the Ce−F stretching vibrations of FBCeF2 are predicted only E

DOI: 10.1021/acs.inorgchem.8b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Structures of the borylene product FBMF2 (M = Sc, Y, La, Ce) optimized using the B3LYP (Roman), CASSCF (italic), and NEVPT2 (bold) functionals. The 6-311++G(3df,3pd) basis set was used for B and F atoms and the SDD pseudopotential basis set for Sc, Y, La, and Ce atoms at the B3LYP level. The QZVP basis set was used for all atoms at the CASSCF level. The TZVP basis set was used for all atoms at the NEVPT2 level. Bond lengths are given in Å and angles in deg. The energies are given in kcal/mol and are relative to the corresponding M + BF3.

3.4 and 6.5 cm−1 higher than observed values, but the deviation of 11B−F the stretching mode is 109.1 cm−1, which is a systematic error for CAS calculations of the B−F mode. The complementary argon matrix experiments were also done with BF3 under the same conditions. As shown in Figure S1, the absorbance bands of product molecules are located at 1336.3, 500.8, and 488.9 cm−1 and at 1379.6, 501.0, and 489.0 cm−1 in the reactions of Ce with 11BF3 and 10BF3, respectively. A mixture of B isotopes has been used to ensure that a single B atom is involved in the product. The spectrum of the reaction of Ce with a mixture of 0.5% 11BF3 and 0.5% 10BF3 (1/3) is shown in Figure S2. In comparison with the spectrum of 0.5% 11BF3 (Figure S2a) and 0.5% 10BF3 (Figure S2c), no additional peaks appear between the 11B−F stretching mode and 10B−F stretching mode, suggesting that a single B atom exists in the product.

Table 6. Compositions of Natural Localized Molecular Orbitals (NLMO) from NBO Analysis of FBMF2 Molecules (M = Sc, Y, La, Ce)a molecule

bond

NLMO

occ

FBScF2

B−Sc σ

80% B(s0.69p0.31) + 20% Sc(s0.34p0.26d0.40) 80% B(s0.69p0.31) + 20% Sc(s0.33p0.24d0.43) 46% B(p) + 54% Sc (p0.14d0.85) 14% B(s0.32p0.68) + 86% F(s0.46p0.54) 14% B(s0.32p0.68) + 86% F(s0.47p0.53) 85% B(s0.69p0.31) + 15% Y(s0.36p0.23d0.41) 84% B(s0.69p0.31) + 16% Y(s0.35p0.21d0.44) 57% B(p0.99) + 43% Y (p0.19d0.81) 14% B(s0.31p0.68) + 86% F(s0.46p0.53) 14% B(s0.31p0.68) + 86% F(s0.47p0.53) 86% B(s0.70p0.30) + 14% La(s0.25p0.18d0.51f0.05) 85% B(s0.69p0.30) + 15% La(s0.24p0.16d0.54f0.06) 57% B(p) + 43% La (p0.11d0.81f0.08) 14% B(s0.32p0.67) + 86% F(s0.47p0.53) 14% B(s0.32p0.67) + 86% F(s0.47p0.52) 85% B(s0.69p0.31) + 15% Ce(s0.26p0.18d0.51f0.05) 84% B(s0.69p0.31) + 16% Ce(s0.25p0.16d0.53f0.06) 55% B(p) + 45% Ce (p0.10d0.72f0.17)

α 0.99

Sc: 1.67

β 0.99

B: 0.31

B−Sc π B−F σ

FBYF2

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B−Y σ

B−Y π

REACTION MECHANISM The reaction proceeds through insertion and then α-F transfer, as previous reports suggested.6,7 The possible reaction products F2BMF, FBMF2, and BMF3 (M = Sc, Y, La, Ce) were all computed at the B3LYP and CCSD(T) levels of theory, but we did not get the stable structure of BMF3. It is interesting to note that the first insertion products of F2BMF are exothermic between 30 and 40 kcal/mol (as shown in Table 5). As shown in Figure 6, the reactions of metals with BF3 give FBMF2, which are exothermic between 40 and 60 kcal/mol. The strong absorptions of F−B symmetric and antisymmetric stretching frequencies of F2BMF are predicted to occur in the 1000−1300 cm−1 region (Table S1), but there are no new product bands observed in this region. In addition, the M−F stretching modes of F2BMF (M = Sc, Y, La, Ce) calculated at 677.4, 606.4, 518.6, and 514.5 cm−1, respectively, are also not observed in experiments. Therefore, the assignments of F2BMF (M = Sc, Y, La, Ce) are ruled out.

B−F σ

FBLaF2

B−La σ

B−La π B−F σ

FBCeF2

B−Ce σ

B−Ce π

F

natural charge

α 1.00 α 1.00

F(B): −0.52

β 1.00

F(Sc): −0.70

α 0.99

Y: 1.72

β 0.99

B: 0.29

α 1.00 α 1.00

F(B): −0.49

β 1.00

F(Y): −0.69

α 0.98

La: 1.73

β 0.98

B: 0.30

α 1.00 α 1.00

F(B): −0.53

β 1.00

F(La): −0.72

α 0.98

Ce: 1.70

β 0.98

B: 0.25

α 1.00

DOI: 10.1021/acs.inorgchem.8b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 6. continued

All of the data are calculated with the B3LYP hybrid density functional. LP denotes lone pair.

(46% B(p) + 54% Sc(p0.14d0.85)). The bonds of doublet state FBYF2 and FBLaF2 molecules are similar to that of FBScF2. However, the triplet FBCeF2 is somewhat different from the group 3 metal complexes, in which the 4f electron of cerium did not participate in bonding on top of one σ bond and a half (p−d) π bond between B and Ce atoms (Figure 7). The orbital occupancies for the FBMF2 (M = Sc, Y, La, Ce) molecules and the effective bond order (EBO) between boron atom and metal atom are summarized in Table 7.

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Table 7. Natural Orbital Occupation Numbers Computed by CASSCF/NEVPT2 for the Active Orbitals of the FBMF2 Molecules (M = Sc, Y, La, Ce)

molecule

bond

NLMO

occ

B−F σ

14% B(s0.32p0.67) + 86% F(s0.47p0.53) 14% B(s0.32p0.67) + 86% F(s0.47p0.52) 100% Ce(d0.01f0.99)

α 1.00

Ce(LP)

natural charge

β 1.00

F(B): −0.51

α 0.99

F(Ce): −0.71

a

MOLECULAR STRUCTURES AND BONDING As shown in Figure 6, the structures of M = Sc, Y, La, Ce complexes were optimized by different methods and possess C2v symmetry with 2A′ ground state. The B−F bond lengths of FBMF2 computed by B3LYP/NEVPT2/CAS are very close to those in FB−MF2 (M = Ti, Zr, Hf, Th).6,7 Note that this bond length slightly increases from Sc to La but decreases from La to Ce (Figure 6). In addition, the calculated FBMF2 molecule has two equal length M−F bonds, which are close to typical group 3 metal−fluorine bond lengths.13 The B−M bond lengths of FB-MF2 calculated by B3LYP/NEVPT2/CAS are longer than a B−M double bond but shorter than a B−M single bond,36 and the B−M bond dissociation energies are estimated to be about 30 kcal/mol (Table 5). For the doublet state FBScF2, a σ bond with an occupation of 1.92 e is formed between B and Sc atoms. In addition, a one-electron (p−d) π type bond with occupation of 0.99 e out of the plane exists by donation from the B atom (most 2p electrons) to the 3d orbital of the Sc atom, which is supported by the 0.56 and 0.43 values of Mulliken spin density on B and Sc atoms, respectively, at the CAS(3e,9o)/Def2-TZVP level.23,37 Natural bond orbital (NBO) analysis by B3LYP hybrid density functional given in Table 6 also indicates that the σ bond mainly originates from the B sp hybridization orbital and the Sc spd hybridization orbital (80% B(s0.69p0.31) + 20% Sc(s0.34p0.26d0.40)). However, the half π type bond is composed of a 2p orbital of a B atom and a 3d orbital of a Sc atom

orbital

FBScF2 (2A′)

FBYF2 (2A′)

FBLaF2 (2A′)

FBCeF2 (3A″)

a1 (σ) a1 (σ*) b1 (π) b1 (π*) b2 (π in pl) b2 (π* in pl) EBO

1.922 0.017 0.987 0.017 0.038 0.0004 1.456

1.922 0.015 0.987 0.013 0.039 0.004 1.458

1.918 0.016 0.987 0.016 0.043 0.0004 1.458

1.605 0.014 0.981 0.016 0.356 0.006 1.453

On our AIM picture18 (Figure 8), the bond critical point between boron and metal in the FBMF2 molecule is located in the region with a positive Laplacian value (∇2ρcp = 0.1369 for B−Sc, 0.1154 for B−Y, 0.0839 for B−La, and 0.0979 for B−Ce), suggesting the depletion of electrons in the bonding region, which corresponds to an ionic interaction (closed shell). The ELF value for a purely ionic bond is very high (nearly 1.0), which is close to the more electronegative atom and is very low (close to 0) in the interstitial region.38 In Figure 9, the ELF values between 0.6 and 0.9 Å in the bonding domains of the B−M (M = Sc, Y, La, Ce) ionic bond are nearly 1.0, while for FB−ZrF2, the ELF value between 0.6 and 0.9 Å is about 0.9, suggesting that a more polarized bond exists in B−M (M = Sc, Y, La, Ce) in comparison to that in

Figure 7. Active molecular orbitals in FBMF2 (M = Sc, Y, La, Ce) at the CAS(3e,9o)/Def2-TZVP level for Sc, Y, and La and CAS(4e, 16o)/Def2TZVP level for Ce. The contour line used is 0.04 e au−3. MO occupation numbers are given with each orbital. G

DOI: 10.1021/acs.inorgchem.8b02801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the ns occupation in the M atom decreases from 33% for the Sc atom to 25% for the La atom, which contributes to the contraction of the ns orbital caused by relativistic effects. Consequently, the bond strength concerning the metal atom is reduced and the M−F stretching mode red-shifts from the Sc to La atom. The change from La atom to a Ce atom is somewhat different, where the stretching mode blue-shifts, which could be due to mainly lanthanide contraction.

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CONCLUSION The reaction products of laser-ablated cerium atoms with boron trifluoride in solid noble matrixes were studied by matrix isolation infrared spectroscopy and theoretical calculations. The reaction products FBMF2 (M = Sc, Y, La, Ce) were produced, which were identified through 10BF3 isotopic shifts and computations based on B3LYP and CASSCF/NEVPT2 density functionals. From Sc to La, the F−B stretching mode red-shifts from 1391.9 cm−1 to 1370.8 and 1337.1 cm−1, but for Ce this mode was observed at 1354.2 cm−1. The high-level computation and NBO analysis suggested that a one-electron π bond and one doubly occupied σ bond exist between the B atom and metal atom. The half π bond is formed by the donation of a valence 2p orbital electron of the boron atom to an empty valence d orbital of the metal atom. However, the σ bond is best described as a highly polarized bond with nearly 2e occupation. The 4f valence electron of the Ce atom located in the inner shell did not participate in bonding in the FBCeF2 complex. The electron localization function (ELF) analysis and the theory of atoms in molecules (AIM) were also applied to investigate the bonding characters.

Figure 8. Contour line diagrams of the Laplacian of the electronic density of FBMF2 (M = Sc, Y, La, Ce) at the B3LYP level, with the basis sets 6-311++G(3df,3pd) used for B and F atoms and SDD used for Sc, Y, La, and Ce atoms. Red lines are in the regions of negative charge concentration (∇2ρ(r) < 0); green lines are in the regions of positive charge depletion (∇2ρ(r) > 0).

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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.8b02801. Infrared spectra and calculated fundamental frequencies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for B.X.: [email protected]. *E-mail for X.W.: [email protected]. ORCID

Figure 9. Sectional line profiles of ELF between the B and M atoms (M = Sc, Y, La, Ce). The B atom corresponds to the 0.0 Å distance position.

Bing Xu: 0000-0002-5521-0035 Zhen Pu: 0000-0003-4608-3609 Xuefeng Wang: 0000-0001-6588-997X Notes

FB−ZrF2, mainly due to the lower electronegativity of group 3 and Ce atoms in comparison with group 4 atoms. The effective bond order (EBO)39 (Table 7) is smaller than that of a double bond in group 4 terminal borylene complexes, which corresponds to bond order 1.5 for the B−M bond in FBMF2 molecules (M = Sc, Y, La, Ce). Note that the oxidation state is +2 for metal and +1 for boron on the basis of natural charges (Table 6). It is interesting to compare frequencies among different metal complexes. In Figure 5, from Sc to La, the F−B stretching mode red-shifts from 1391.9 cm−1 to 1370.8 and 1337.1 cm−1. Moreover, for the FBMF2 molecule the antisymmetric and symmetric stretching modes of M−F red-shift also, where 693.4 and 629.7 cm−1 observed for ScF2 shift to 579.6 and 569.9 cm−1 for YF2, and 499.9 and 513.0 cm−1 for LaF2. As shown in Table 6,

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 21371136 and 21873070).

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REFERENCES

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