Infrared Spectroscopic and Theoretical Studies on the OMF2 and OMF

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Infrared Spectroscopic and Theoretical Studies on the OMF and OMF (M=Cr, Mo, W) Molecules in Solid Argon 2

rui wei, Qingnuan Li, Yu Gong, Lester Andrews, Zongtang Fang, Kanchana Sahan Thanthiriwatte, Monica Vasiliu, and David A Dixon J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08088 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 21, 2017

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The Journal of Physical Chemistry

Infrared Spectroscopic and Theoretical Studies on the OMF2 and OMF (M=Cr, Mo, W) Molecules in Solid Argon Rui Wei,a,b Qingnuan Li,a Yu Gong,*,a Lester Andrews,c Zongtang Fang,d K. Sahan Thanthiriwatte,d Monica Vasiliu,d and David A. Dixon,*,d a

Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of

Sciences, Shanghai 201800, China b

University of Chinese Academy of Sciences, Beijing 100049, China,

c

Department of Chemistry, University of Virginia, Charlottesville, VA, 22904-4319, United

States d

Department of Chemistry, The University of Alabama, Tuscaloosa, AL, 35487-0336, United

States Abstract Group 6 metal oxide fluoride molecules in the form of OMF2 and OMF (M=Cr, Mo, W) have been prepared via the reactions of laser-ablated metal atoms and OF2 in excess argon. Product identifications were carried out by using infrared spectroscopy,

18

OF2 sample and electronic

structure calculations. Reactions of group 6 metal atoms and OF2 resulted in the formation of ternary OCrF2, OMoF2 and OWF2 molecules with C2v symmetry in which the tetravalent metal center is coordinated by one oxygen and two fluorine atoms. Both OCrF2 and OMoF2 are computed to possess triplet ground states and a closed shell singlet is the ground state for OWF2. Triatomic OCrF, OMoF and OWF molecules were also observed during sample deposition. All

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three molecules were computed to have a bent geometry and quartet ground state. A bonding analysis showed that the OMF2 molecules have highly ionic M-F bonds. 3OCrF2 and 3OMoF2 have a M-O double bond composed of a σ bond and a π bond. 1OWF2 has a M-O triple bond consisting of a σ bond, a π bond and a highly delocalized O lone pair forming the other π bond. The M-O bonds in the OMF compounds have triple bond character for all three metals.

Corresponding authors: Email: [email protected] (Y.G.), [email protected] (D.A.D.).


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Introduction As important members of transition metal oxide fluorides, group 6 metal oxide fluorides such as MO2F2 and MOF4 have been known as strong oxidizing agents for decades.1 The geometric and electronic structures of MO2F2 especially CrO2F2 in solid, liquid and gas phases have been investigated by different spectroscopic methods 2 , 3 , 4 , 5 , 6 , 7 and theoretical calculations,8,9,10 which revealed that all these species are pseudo-tetrahedral with C2v symmetry and terminal M-O and M-F bonds. For the other oxofluoride MOF4 with M in the +VI formal oxidation state, both experimental and theoretical studies have confirmed that it should possess a square pyramidal structure with C4v symmetry.10,11,12,13,14,15 Compared with these well-known oxofluorides with a +VI oxidation state metal center, little is known about the structure of group 6 oxofluorides with the metal in a lower oxidation state.1 Recently, it has been demonstrated that tetravalent OMF2 (Ti, Zr, Hf, Ce, U, Th) molecules with the metal in the formal +IV oxidation state can be readily formed via the reactions of the relevant metal atoms with OF2 in an argon matrix. 16,17 , 18 Based on the experimental and theoretical results, all of these molecules were characterized to have C2v or pyramidal Cs symmetries with terminal M-O triple bonds and M-F bonds. The successful preparation and identification of such complexes suggest the possibility of synthesizing additional OMF2 molecules with metals from other columns of the Periodic Table. We report the preparation of group 6 OMF2 molecules with a metal center in the +IV oxidation state via the reactions of laser-ablated group 6 metal atoms and OF2 in excess argon. All of these products are



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characterized by infrared spectroscopy including the use of isotopic labeling using

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OF2 and

electronic structure calculations. Evidence for the formation of triatomic OMF molecules is also provided, where the metal center is in the +III oxidation state. Experimental and computational methods Experimental The preparation and characterization of the product molecules in excess argon were carried out on the experimental apparatus described previously.19,20 The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a chromium or molybdenum or tungsten target (Johnson Matthey) mounted on a rotating stainless steel rod. These laser-ablated metal atoms were codeposited with 3−4 mmol of argon (Matheson, research) containing 1.0% OF2 (Ozark-Mahoning) onto a CsI cryogenic window for 60 min. The 18

OF2 sample (91%

18

O enriched) was synthesized and kindly provided by Arkell and

co-workers.21 Both OF2 and

18

OF2 samples were cooled to 77 K using liquid N2 and evacuated

to remove residual N2 and O2 before use. FTIR spectra were recorded at 0.5 cm−1 resolution on a Nicolet 750 FTIR instrument with a HgCdTe range B detector. Matrix samples were annealed at different temperatures and cooled back to 4K for spectral acquisition. Selected samples were subjected to broadband (λ > 220nm) irradiation by a medium-pressure mercury arc street lamp (Philips, 175W) with the outer globe removed. Computational The initial structures and vibrational frequencies of the MOF2, and OMF molecules were calculated at the density functional theory level using using the B3LYP hybrid functional,22,23 the BP86 functional, and the PW91 functional, with the aug-cc-pVDZ basis set


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for O and F 24 and aug-cc-pVDZ-PP basis sets for the metals with the accompanying pseudopotential25,26,27,28 using Gaussian 09.29Atomic orbital occupancies were determined using NBO630,31 for the natural bond orbital (NBO)32,33,34,35 population analysis at the DFT level. Subsequent geometry optimizations and frequency calculations starting from the DFT geometries were done at the at the coupled cluster R/UCCSD(T)36,37,38,39,40,41,42 levels with the same basis set and ECP. Only the valence electrons were correlated. In the R/UCCSD(T) approach, a restricted open shell Hartree-Fock (ROHF) calculation is initially performed and the spin constraint was then relaxed in the coupled cluster calculation. The CCSD(T) calculations were done with MOLPRO.43,44 Results and discussion The infrared spectra from the reactions of laser-ablated group 6 metal atoms and OF2 in excess argon are shown in Figures 1-4. An intense absorption at 1028.2 cm−1 due to the diatomic OF molecule was observed in all the experiments.45 This band appeared upon sample deposition, during which OF2 dissociation occurred as a result of ultraviolet irradiation from the laser ablated plasma plume.46 The OF absorption decreased on annealing and increased when the sample was subjected to broad band irradiation (λ > 220nm). Other weak absorptions due to O3, CO2 and SiF4 were present in the spectra right after sample deposition as well, which were common species in the spectra from the reactions of OF2 and laser-ablated metal atoms.16,17,18,47,48,49 All of the new product bands observed in the experiments with OF2 and 18OF2 samples are listed in Table 1.



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Chromium Reactions Reactions of laser-ablated chromium atoms and OF2 resulted in the formation of a series of chromium dependent products (Figure 1). Binary chromium oxide and fluoride molecules such as CrO2, CrF2, CrF3 and CrF4 were observed at 965.3, 654.4, 749.2 and 784.2 cm−1.50,51,52 A set of absorptions at 1010.3, 1001.7, 782.1 and 721.3 cm−1 was observed in the spectra after sample deposition. The intensities of these peaks remained unchanged until the sample was annealed to 30K. Both the band positions and relative intensities agree well with the 1013.9, 1006.7, 784.8 and 721.7 cm−1 absorptions observed in N2 matrix which were assigned to the CrO2F2 molecule.4 The weak band at 746.7 cm−1 should be due to CrOF4 based on previous experimental results.12,51 In addition to these well-known ternary oxide fluoride molecules, new product absorptions were observed at 1017.3, 735.3, 662.1, 935.9 and 646.0 cm−1 (Figure 1, traces a-e). The first three absorptions slightly increased when the sample was annealed to 20K but decreased after 30K annealing. Both sample annealing and broad band irradiation had little effect on the intensities of the 935.9 and 646.0 cm−1 bands. To help identify the products of the reaction of the metals with OF2 in an argon matrix all of the experiments were repeated by using the

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OF2 sample under similar conditions. The

absorption due to the M-O or M-F stretches perturbed by oxygen exhibited certain

18

O shift

depending on the nature of the vibrational mode. The infrared spectra from the reactions of chromium and

18

OF2 in excess argon are shown in Figure 1, traces f-j. All of the chromium

dependent absorptions above 900 cm-1 exhibited isotopic shifts of 35-40 cm−1 while the shift for the absorptions below 800 cm−1 was no more than 6 cm−1. Cr18O2


50

was formed in the

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experimentwith 18OF2 and no shift was observed for the CrF2, CrF3 and CrF4 molecules.



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Table 1. Infrared Absorptions (cm-1) for the New Products from the Reactions of Group 6 Metal Atoms with 16OF2 and 18OF2 in Excess Argon and the Calculated Infrared Absorptions (cm-1) and IR Intensities (km/mol) at PW91/aug-cc-pVDZ(-PP) and CCSD(T)/aug-cc-pVDZ(-PP) Levels. 16 18 Molecule 16OF2 18OF2 Isotopic 16OF2 PW91 18OF2 PW91 Isotopic OF2 OF2 Isotopic Assignmenta (expt) (expt) ratios ratios CCSD(T) CCSD(T) ratios (expt) PW91 CCSD(T) 3 OCrF2 1017.3 976.5 1.0418 1064.6(161) 1020.3(156) 1.0434 1015.5 974.1 1.0425 Cr-O stretch 735.3 735.3 1.0000 734.7(198) 734.7(197) 1.0000 765.9 765.9 1.0000 anti F-Cr-F stretch 662.1 659.9 1.0033 660.5(54) 658.2(51) 1.0035 685.4 682.5 1.0042 sym F-Cr-F stretch 3 92 OMoF2 1007.6 959.2 1.0505 1014.8(142) 965.8(134) 1.0507 1013.5 964.0 1.0513 Mo-O stretchb b b 94 1013.2(141) 964.1(133) 1.0509 1012.0 962.4 1.0515 Mo-O stretch b b 95 1012.4(140) 963.2(132) 1.0511 1011.3 961.7 1.0516 Mo-O stretch b b 96 1011.6(140) 962.4(131) 1.0511 1010.5 960.9 1.0516 Mo-O stretch b b 97 1010.9(139) 961.6(130) 1.0513 1009.9 960.2 1.0518 Mo-O stretch 98 1003.3 954.4 1.0512 1010.1(138) 960.8(130) 1.0513 1009.2 959.5 1.0518 Mo-O stretch 100 1001.9 952.9 1.0514 1008.7(137) 959.2(129) 1.0516 1007.9 958.0 1.0521 Mo-O stretch c 652.3 652.9(179) 652.9(179) 1.0000 676.8 676.7 1.0001 anti F-92Mo-F stretchd d d 651.2(178) 651.2(178) 1.0000 675.1 675.0 1.0001 anti F-94Mo-F stretch d d 650.4(177) 650.4(177) 1.0000 674.3 674.2 1.0001 anti F-95Mo-F stretch d d 649.6(177) 649.6(177) 1.0000 673.5 673.4 1.0001 anti F-96Mo-F stretch d d 648.8(176) 648.8(176) 1.0000 672.7 672.7 1.0000 anti F-97Mo-F stretch 647.5 647.5 1.0000 648.0(176) 648.0(175) 1.0000 672.0 671.9 1.0001 anti F-98Mo-F stretch 646.0 646.0 1.0000 646.5(174) 646.5(174) 1.0000 670.5 670.5 1.0000 anti F-100Mo-F stretch e e 635.4(67) 634.9(65) 1.0008 661.2 661.1 1.0002 sym F-92Mo-F stretch e e 634.9(67) 634.4(65) 1.0008 660.5 660.4 1.0002 sym F-94Mo-F stretch e e 634.6(67) 634.2(65) 1.0006 660.2 660.1 1.0002 sym F-95Mo-F stretch 633.4 633.1 0.3 634.4(67) 633.9(65) 1.0008 659.9 659.8 1.0002 sym F-96Mo-F stretch e e 634.1(67) 633.7(66) 1.0006 659.6 659.5 1.0002 sym F-97Mo-F stretch e e 633.9(67) 633.4(66) 1.0008 659.3 659.2 1.0002 sym F-98Mo-F stretch 8 ACS Paragon Plus Environment

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1

OWF2

4

OCrF

4

OMoF

4

a

OWF

e

e

1033.7 700.3

980.3 700.3

f

f

935.9 646.0 989.3 987.3 986.3 985.3 984.3 983.5 981.6

900.3 641.5 942.9 940.7 939.8 938.6 937.9 936.9 935.3

f

f

f

f

f

f

f

f

f

f

f

f

f

f

994.6 622.6

942.7 622.6

1.0545 1.0000 1.0395 1.0070 1.0492 1.0495 1.0495 1.0498 1.0495 1.0497 1.0495

1.0551 1.0000

633.4(67) 1040.2(84) 708.5(122) 691.0(35) 995.1(174) 666.9(92) 982.5(151) 981.0(149) 980.2(149) 979.5(148) 978.7(148) 978.0(147) 976.6(146) 634.6(108) 633.6(108) 633.1(108) 632.6(108) 632.1(108) 631.6(108) 630.7(108) 986.8(113) 620.2(116)

633.0(66) 985.6(76) 708.2(122) 691.0(35) 954.6(168) 664.1(87) 934.9(141) 933.3(139) 932.5(139) 931.7(138) 930.9(137) 930.1(137) 928.7(136) 634.1(106) 633.1(106) 632.6(106) 632.1(106) 631.7(106) 631.2(106) 630.3(106) 934.8(102) 620.2(116)

1.0006 1.0554 1.0004 1.0000 1.0424 1.0042 1.0509 1.0511 1.0512 1.0513 1.0513 1.0515 1.0516 1.0008 1.0008 1.0008 1.0008 1.0006 1.0006 1.0006 1.0556 1.0000

658.7 1033.3 747.8 725.0 907.6 651.9 968.5 966.9 966.2 965.5 964.7 964.0 962.7 650.5 649.6 649.1 648.7 648.1 647.7 646.8 973.4 652.8

658.7 979.1 747.7 725.0 875.6 644.9 921.6 919.9 919.1 918.3 917.6 916.9 915.4 650.1 649.1 648.7 648.2 647.8 647.3 646.5 922.1 652.8

1.0000 1.0554 1.0001 1.0000 1.0365 1.0109 1.0509 1.0511 1.0512 1.0514 1.0513 1.0514 1.0517 1.0006 1.0008 1.0006 1.0008 1.0005 1.0006 1.0005 1.0556 1.0000

sym F-100Mo-F stretch W-O stretch anti F-W-F stretch sym F-W-F stretch Cr-O stretch Cr-F stretch 92 Mo-O stretch 94 Mo-O stretch 95 Mo-O stretch 96 Mo-O stretch 97 Mo-O stretch 98 Mo-O stretch 100 Mo-O stretche 92 Mo-F stretch 94 Mo-F stretch 95 Mo-F stretch 96 Mo-F stretch 97 Mo-F stretch 98 Mo-F stretch 100 Mo-F stretch W-O stretch W-F stretch

anti=antisymmetric; sym=symmetric. b partially resolved absorptions due to nMo-O (n = 94-97) stretches: 1006.1, 1005.4, 1004.7

cm-1 (16OF2); 957.4, 956.7, 955.8 cm-1 (18OF2). c covered by the C18O2 absorption. d partially resolved absorptions due to F-nMo-F (n = 94-97) stretches: 649.7 cm-1 (16OF2, 18OF2). e Mo isotopic splitting not resolved. f not observed.



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Figure 1. Infrared spectra of laser-ablated chromium atoms co-deposited with OF2 in excess argon at 4K: (a) Cr + 1% 16OF2/Ar co-deposition for 60 min; (b) after annealing to 20 K; (c) after broad band (λ > 220 nm) irradiation; (d) after annealing to 30K; (e) after annealing to 35K; (f) Cr + 1%

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OF2/Ar co-deposition for 60 min; (g) after annealing to 20 K; (h) after 10 ACS Paragon Plus Environment

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broad band (λ > 220 nm) irradiation; (i) after annealing to 30K; (j) after annealing to 35K. The asterisk denotes the SiF4 band. C16,18O2 denotes the absorptions due to C16O2, C16,18O2 and C18O2. F denotes an unassigned band observed upon reactions of Cr and F2 in solid argon as well.

OCrF2 The 1017.3, 735.3 and 662.1 cm−1 absorptions were not observed when chromium reacted with O2 or F2 in argon matrix,50,51,52 and they were absent in previous spectroscopic studies of CrO2F2 and CrOF4 either.4,12,51 The identical behavior of the three absorptions during sample annealing suggests that they should be due to different vibrational modes of a new ternary chromium oxide fluoride molecule. The 1017.3 cm−1 band shifted to 976.5 cm−1 with an

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O/18O ratio of 1.0418, suggesting it is a terminal Cr-O stretch but coupled with

another vibrational mode as in the case of CrOCrO.53 A small shift of 2.2 cm−1 was observed for the 662.1 cm−1 band while the more intense 735.3 cm−1 band revealed no shift upon 18O substitution. Both absorptions appeared in a region where chromium fluorides absorb,51,52 and one of them is perturbed by oxygen. Since no other absorption tracked these bands in the same region, we assign the 662.1 and 735.3 cm−1 bands to the symmetric and antisymmetric F-Cr-F stretches of a new molecule. Based on the observation of one terminal Cr-O and two F-Cr-F stretches, the new molecule is identified as OCrF2. The lack of 18O shift for the 735.3 cm−1 band is due to the low participation of oxygen in this antisymmetric vibrational mode, as in the cases of OScF2 and OTiF2.18,48 The calculations for 3OCrF2 molecules yield three infrared active absorptions above 400 cm-1 (Table 1). The PW91 Cr-O stretch is about 50 cm-1 above the experimental value 11 ACS Paragon Plus Environment


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and the two Cr-F stretches are slightly smaller within a few cm-1 of the experimental value. The CCSD(T) CrO stretch is about 2 cm-1 smaller than experiment and the Cr-F stretches are about 20-30 cm-1 larger than the experimental values. Because the computational values are harmonic frequencies and the experimental values include an anharmonic correction, we would expect the computed values to be larger than experiment. Thus the CCSD(T) value is probably too low for the Cr-O stretch and in contrast, the PW91 Cr-F stretches are too low. The computed isotopic

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O/18O ratios for the three stretches are in good agreement with the

experimental values further confirming the assignment of the bands to OCrF2. OCrF The new product absorptions at 935.9 and 646.0 cm−1 appeared together with the formation of OCrF2. The first band shifted to 900.3 cm-1 with an

16

O/18O ratio of 1.0395,

which is close the value of the antisymmetric O-Cr-O stretch.50 Since the contains 9%

16

18

OF2 sample

OF2, a triplet splitting with relative intensities of (0.91)2:2×(0.91)

×(0.09):(0.09)2 would be observed if two equivalent oxygens were involved in this mode. However, a doublet at 900.3 and 935.9 cm−1 with approximately 10:1 relative intensities was observed when

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OF2 was used (Figure 1, traces f-j), suggesting the participation of one

oxygen atom in this mode. As a result, the 935.9 cm−1 band should be due to a terminal Cr-O stretch. In the lower region, 18O substitution resulted in a small shift of 4.5 cm−1 for the 646.0 cm−1 band. No other absorption tracked this band in the same region. Based on these experimental observations, the 935.9 and 646.0 cm−1 absorptions are assigned to the Cr-O and Cr-F stretches of the triatomic OCrF molecule. The 16O/18O isotopic frequency ratio for the terminal Cr-O stretch is lower than that of the diatomic CrO molecule (1.0453) owing to the coupling between Cr-O and Cr-F stretches, which is consistent with the 4.5 cm-1 18O shift 12 ACS Paragon Plus Environment

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observed for the Cr-F stretch. In the case of OCrF2, the Cr-O and Cr-F stretches are less coupled such that 18O substitution resulted in a higher ratio for the Cr-O stretch (1.0418) and a smaller shift for the symmetric F-Cr-F stretch (2.2 cm-1). CCSD(T) calculations on 4OCrF resulted in a Cr-O stretching frequency about 30 cm-1 below the experimental value and the Cr-F stretch is about 6 cm-1 higher than experiment. The PW91 Cr-O stretch is predicted to be about 60 cm-1 above experiment and the Cr-F is about 20 cm-1 greater than experiment, which supports our assignment of the OCrF molecule. The calculated

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O/18O ratios for the Cr-O and Cr-F stretches are slightly

lower and higher than the experimental values respectively at CCSD(T) level while a slightly higher Cr-O ratio and lower Cr-F ratio was obtained using PW91 functional. Apparently, the coupling between the calculated Cr-O and Cr-F stretches depends on the level of theory. Molybdenum Reactions Figure 2 shows the infrared spectra of products formed upon reactions of molybdenum and OF2 in solid argon. The broad feature around 1044 and 707 cm−1 increased on sample annealing and broad band irradiation, and they have been assigned to the Mo-O and Mo-F stretches of the MoOF4 molecule.14 Absorptions due to binary molybdenum oxide and fluoride molecules were barely observed.54,55 New major product absorptions were observed at 1007.6, 1006.1, 1005.4, 1004.7, 1003.3, 1001.9, 652.3, 649.7,



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Figure 2. Infrared spectra of laser-ablated molybdenum atoms co-deposited with OF2 in excess argon at 4K: (a) Mo + 1% 16OF2/Ar co-deposition for 60 min; (b) after annealing to 20 K; (c) after broad band (λ > 220 nm) irradiation; (d) after annealing to 30K; (e) after annealing to 35K; (f) Mo + 1% 18OF2/Ar co-deposition for 60 min; (g) after annealing to 20 K; (h) after broad band (λ > 220 nm) irradiation; (i) after annealing to 30K; (j) after annealing 14 ACS Paragon Plus Environment

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to 35K. The asterisk denotes the SiF4 band. C16,18O2 denotes the absorptions due to C16O2, C16,18O2 and C18O2. F denotes an unassigned band observed upon reactions of Mo and F2 in solid argon as well.

Figure 3. Expanded wavenumber spectra for the product absorptions with resolved natural Mo isotopes (92Mo,

94

Mo,

95

Mo,

96

Mo,

97

Mo,

98

Mo,

100

Mo) compared with line spectra

showing absorbances based on the percent natural abundance of each isotope.

647.5, 646.0 and 633.4 cm−1 (Figure 2, traces a-e), which decreased when the sample was annealed to 30K. The patterns of the peaks between 1007.6 and 1001.9 as well as 652.3 and 15 ACS Paragon Plus Environment


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646.0 cm−1 match well with the mass distributions of natural molybdenum isotopes (92Mo, 94

Mo, 95Mo, 96Mo, 97Mo, 98Mo, 100Mo) although the peaks arising from

94

Mo, 95Mo, 96Mo and

97

Mo were not well resolved (Figure 3). No molybdenum isotope was resolved for the 633.4

cm−1 absorption, but a shoulder at 634.5 cm−1 was observed on the left side of this band. Note that the width of the 633.4 cm−1 band is much smaller than that of the 652.3-646.0 cm−1 band group in the same region. Another new set of weak absorptions at 989.3, 987.3, 986.3, 985.3, 984.3, 983.5 and 981.6 cm−1 appeared after sample deposition, and their intensities remained almost unchanged throughout the experiments. The overall profile of these bands fits the mass distributions of natural molybdenum isotopes (Figure 3). Figure 2 (traces f-j) show the infrared spectra from the reactions of molybdenum atoms with

18

OF2 in excess argon. The newly observed molybdenum product absorptions

above 930 cm−1 red-shifted by about 50 cm−1. For the absorptions below 740 cm−1, only the 633.4 cm−1 band had a small 18O shift of 0.3 cm−1 (Figure 2) while no shift was observed for the remaining absorptions. OMoF2 Most of the new product absorptions in Figure 2 showed isotopic splittings arising from the natural molybdenum isotopes (92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, 100Mo). For the two sets of absorptions at 1007.6, 1006.1, 1005.4, 1004.7, 1003.3, 1001.9 and 652.3, 649.7, 647.5, 646.0 cm−1, their band profiles are very similar with the mass distributions of natural molybdenum isotopes although only the isotopic bands of

92

Mo, 98Mo and 100Mo are clearly

resolved, suggesting they are two different vibrational modes of the same molecule involving one Mo atom. The other band at 633.4 cm−1 also tracked the above two sets of absorptions, and a shoulder at 634.5 cm-1 is the only Mo isotopic absorption resolved. Experiments with 16 ACS Paragon Plus Environment

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18

OF2 sample revealed isotopic ratios around 1.0510 for the peaks between 1007.6 and 1001.9

cm−1, in agreement with the value of diatomic MoO molecule.54 The 633.4 cm−1 band shifted to 633.1 cm−1 upon 18O substitution while no shift was observed for the absorptions between 652.3 and 646.0 cm−1. Based on the band position as well as 18O shift, the 633.4 cm−1 band as well as the 652.3, 649.7, 647.5 and 646.0 cm−1 band group should be due to the symmetric and antisymmetric F-Mo-F stretches of the new molecule. Hence, the absorptions at 1007.6, 1006.1, 1005.4, 1004.7, 1003.3, 1001.9, 652.3, 649.7, 647.5, 646.0 and 633.4 cm−1 are assigned to the OMoF2 molecule. The absence of notable splittings for the 633.4 cm-1 band is consistent with the fact that Mo is less involved in the symmetric F-Mo-F stretch than the antisymmetric one, similar to the case of OZrF2.18 The analysis of the Mo results is complicated by the presence of many isotopes with comparable amounts from

92

Mo to

100

Mo. At the CCSD(T) level, the calculated Mo-O

vibrational frequencies are 5 to 10 cm-1 above the experimental values and the Mo-F stretches are about 25 to 30 cm-1 above the experimental values. In this case, the PW91 values are in excellent agreement with the CCSD(T) values for the Mo-O stretches. The PW91 results for the Mo-F stretches are about 15 cm-1 lower than the CCSD(T) values and in almost prefect agreement with experiment. The 18O shifts for the M-O and F-M-F stretches of OMoF2 agree well with the experimental results (Table 1). In addition, the computed widths of the isotopic band groups (92Mo-100Mo) for the Mo-O and antisymmetric F-Mo-F stretches are also consistent with the observed width of 5.7 and 6.3 cm-1. The symmetric F-Mo-F stretch exhibited small band width and the natural Mo isotopes were barely resolved (Figure 3), which is in line with the calculation results. 17 ACS Paragon Plus Environment


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OMoF As observed for the Mo-O stretch of the OMoF2 molecule, a series of absorptions at 989.3, 987.3, 986.3, 985.3, 984.3, 983.5 and 981.6 cm−1 also exhibited splittings similar to those of natural Mo isotopes (Figures 2 and 3). The Mo isotopic pattern is indicative of the participation of single Mo atom in this mode. The width of the band group (92Mo-100Mo) is 7.7 cm−1, larger than that for the OMoF2 molecule (5.7 cm−1), suggesting molybdenum is more involved in this vibrational mode than in the Mo-O stretch of OMoF2. In the experiment with 18OF2 sample, these absorptions shifted to 942.9, 940.7, 939.8, 938.6, 937.9, 936.9 and 935.3 cm−1 with 16O/18O ratios around 1.0495, close to the value of diatomic MoO molecule54 as well as the Mo-O stretch of OMoF2. In the lower region where molybdenum fluorides absorb,55 no peak that tracked these absorptions was detected. Since none of the 989.3, 987.3, 986.3, 985.3, 984.3, 983.5 and 981.6 cm−1 absorption was observed when molybdenum reacted with O2 in argon matrix,54 they should belong to a new molybdenum oxide fluoride molecule. We assign these bands to the terminal Mo-O stretch of triatomic OMoF molecule, the Mo-F stretch of which is probably too weak to be detected due to isotopic dilution arising from Mo isotopes. The CCSD(T) calculations resulted in a Mo-O stretching frequency about 20 cm-1 below the experimental for OMoF and the PW91 value is about 7 cm-1 smaller. The 16O/18O ratio for the Mo-O stretch is in good agreement with the experimental ratio. The Mo-O stretch is about 50% more intense than the Mo-F stretch. Given the weak intensity of the observed Mo-O stretch, it is unlikely to detect the weak Mo-F band owing to the presence of seven natural Mo isotopes by which the overall intensity of the Mo-F stretching vibration is significantly reduced. The computed width of this band group (92Mo-100Mo) for the Mo-O 18 ACS Paragon Plus Environment

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stretch is in good agreement with the experimental width of 7.7 cm-1, which further confirms our assignment of the OMoF molecule even though the Mo-F stretch was not detected. Tungsten Reactions Laser-ablated tungsten atoms reacted with OF2 to form a series of products as well, which are shown in Figure 4 (traces a-e). The binary WF6 molecule appeared upon sample deposition as evidenced by the absorption at 706.9 cm−1.55 In addition, ternary WOF4 molecule was observed at 689.4 and 1052.8 cm−1, the structure of which has been well defined.14 The absorptions for both molecules increased as the sample was annealed to higher temperature at which aggregations of WOF4 were formed as revealed by the broad features that were a few cm-1 lower. In addition to these known molecules, two sets of new product absorptions were observed in the infrared spectra. The first set at 1033.7 and 700.3 cm−1 increased on broad band irradiation and decreased upon sample annealing to 35K. The other set of absorptions was observed at 994.6 and 622.6 cm−1. Their intensities remained unchanged until the sample was annealed to 35K when both peaks decreased.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Infrared spectra of laser-ablated tungsten atoms co-deposited with OF2 in excess argon at 4K: (a) W + 1% 16OF2/Ar co-deposition for 60 min; (b) after annealing to 20 K; (c) after broad band (λ > 220 nm) irradiation; (d) after annealing to 30K; (e) after annealing to 35K; (f) W + 1%

18

OF2/Ar co-deposition for 60 min; (g) after annealing to 20 K; (h) after

broad band (λ > 220 nm) irradiation; (i) after annealing to 30K; (j) after annealing to 35K. 20 ACS Paragon Plus Environment

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Expanded scale infrared spectra (f-j) are shown in Figure S4 in Supporting Information. The asterisk denotes the SiF4 band. C16,18O2 denotes the absorptions due to C16O2, C16,18O2 and C18O2. F denotes an unassigned band observed upon reactions of W and F2 in solid argon as well.

Figure 4 (traces f-j) show the infrared spectra from the reactions of tungsten atoms with

18

OF2 in excess argon. The newly observed tungsten product absorptions above 930

cm−1 red-shifted by about 50 cm−1. For the absorptions below 740 cm−1, no shift was observed for the remaining absorptions. OWF2 Following the example of OCrF2 and OMoF2, the 1033.7 and 700.3 cm−1 absorptions observed in the reactions of W and OF2 are assigned to the W-O and F-W-F stretches of the OWF2 molecule. The 1033.7 cm−1 band shifted to 980.3 cm−1 with an 16O/18O frequency ratio of 1.0545, which is characteristic of a terminal W-O stretch.54,56 The 700.3 cm−1 band exhibited no 18O shift, and it is assigned to the antisymmetric F-W-F stretch of OWF2 based on its band position. For 1OWF2, the CCSD(T) W-O stretch is essentially the same as the experimental value and the PW91 stretch is about 6 cm-1 higher. The antisymmetric F-W-F stretch at the CCSD(T) level is computed to be about 50 cm-1 above experiment. The PW91 antisymmetric F-W-F stretch is only 6 cm-1 above experiment. The values for 3OWF2 are given in the Supporting Information and are not as consistent with the experimental values as are those for the singlet. Both the calculated band positions as well as the isotopic ratios for the W-O and F-W-F antisymmetric stretches of OWF2 are in good agreement with experimental values. 21 ACS Paragon Plus Environment


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The symmetric F-W-F stretching vibrational frequency of OWF2 was predicted at 725 cm-1 at the CCSD(T) level. If the experimental position of this mode were 40-50 cm-1 lower than the computed value as obtained for the antisymmetric F-W-F mode, a weak band around 675-685 cm-1 would be observed. However, this region is dominated by the broad band due to WOF4 as well as its aggregations, which makes it less likely for the symmetric F-W-F mode to be observed. A similar conclusion can be obtained with the PW91 frequencies. OWF The newly observed 994.6 and 622.6 cm−1 absorptions in Figure 4 are assigned to the W-O and W-F stretches of the OWF molecule. The

16

O/18O ratio of the 994.6 cm−1 band is

1.0551, characteristic of a terminal W-O stretch.54,56 The W-F stretch at 622.6 cm−1 exhibited no 18O shift, suggesting W-O and W-F stretches are barely coupled in this triatomic molecule. For 4OWF, the CCSD(T) value for the W-O stretch is again about 20 cm-1 below the experimental value and the W-F stretch is about 30 cm-1 above the experimental value. In contrast, the PW91 value for the W-O stretch is about 8 cm-1 too low as compared to experiment and the W-F stretch is just below the experimental value by only a few cm-1. The 16

O/18O ratios for the W-O and W-F stretches agree well with the experimental isotopic

ratios. Electronic Structure Calculations To get a further insight into the geometric and electronic structures of these new product molecules, electronic structure calculations were carried out at the DFT and CCSD(T) levels of theory. Both OCrF2 and OMoF2 were computed to have 3

A2 ground states in C2v symmetry. The S-T difference (∆H(0K)) for Cr is 22.5(22.2)

kcal/mol at the CCSD(T)/aug-cc-pvDZ-PP(PW91/aug-cc-pvDZ-PP) level and for Mo is 12.4(12.8) kcal/mol at the same level. In contrast, OWF2 has a closed shell 1A1 ground state 22 ACS Paragon Plus Environment

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with the 3A2 state being 3.8(1.8) kcal/mol higher in energy at the same CCSD(T) level. The Cr-O bond distance (Table 2) is significantly shorter than the Mo-O and W-O bond distances which are comparable. The same is true for the longer M-F distances. The ∠FMO is ~ 120°. Geometry optimizations on OCrF, OMoF and OWF resulted in a bent 4A' state. The M-O bond distance increases in OMF as compared to OMF2 with the largest change predicted for Cr and the smallest for W. The M-F bond distances also increase in OMF but not by as much as the M-O, especially for Cr. The ∠FMO does not show any periodic behavior with angle largest for Cr and smallest for Mo, all near 150 ± 5°. Table 2. Optimized Geometries for OMF2 (C2v) and OMF (Cs) CCSD(T)/aug-cc-pVDZ(-PP)

PW91/aug-cc-pVDZ(-PP)

Molecule

M-O (Å)

M-F (Å)

∠FMO (°) M-O (Å)

M-F (Å)

∠FMO (°)

3

OCrF2

1.569

1.764

120.2

1.562

1.752

121.1

3

OMoF2

1.684

1.906

121.7

1.682

1.895

121.3

3

OWF2

1.703

1.907

121.4

1.697

1.897

120.6

1

OWF2

1.690

1.857

112.9

1.685

1.845

113.2

4

OCrF

1.626

1.798

155.5

1.592

1.769

140.0

4

OMoF

1.706

1.920

145.7

1.695

1.900

140.7

4

OWF

1.719

1.925

151.3

1.708

1.910

145.6

Orbital occupancies were calculated using NBO630,31 for the natural bond orbital (NBO)32,33,34,35 natural population analysis (NPA). The results with are in the Supporting Information. The results from the BP86 and PW91 functionals are identical and do not differ much from the B3LYP results. B3LYP gives slightly more positive charges on the metal than do BP86 and PW91. For 3OMF2, Cr is the least positive metal. However, the charge on W in 23 ACS Paragon Plus Environment


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1

OWF2, the lowest energy structure for W, is comparable to that for Cr in 3OCrF2. For the

triplets, the charge transfer is between the O and the M as the fluorine charges do not change by much. For OMF, the charges on the three different metals are comparable with differences less than 0.1e. With BP86, the excess spin in the triplets gives a d2 configuration for OCrF2, a 5s0.24d1.8 configuration for OMoF2 and a 6s0.35d1.7 configuration for OWF2. The most d character is for OCrF2 consistent with the least amount of excess positive charge on the Cr as compared to the Mo and W. For 4OMF, the excess spin on the metal is split between the s and d orbitals and gives configurations of 4s0.43d2.8, 5s0.64d2.4, and 6s0.75d2.4 for M= Cr, Mo, and W respectively. For both OMF2 and OMF, the populations of Cr 4s, Mo 5s, and W 6s orbitals follows the order of Cr < Mo < W. The orbital compositions in OMF2 and OMF compounds from the NBO analysis are shown in the Supporting Information. The M-F single bonds in OMF2 are highly ionic. The open shell orbitals on the M for Cr and Mo are one d orbital and one orbital with some s character, with the s character being larger on Mo than on Cr. For 1OWF2, the doubly occupied lone pair has significantly more s character, almost one-third. The M-O bonds in all of the OMF2 (M = Cr, Mo, W) compounds have substantially more density on O than on the metal so the M-O bonds are highly ionic. The BP86 and B3LYP functionals generally give consistent results with the exception of the Cr-O bond in 3OCrF2. For 3OCrF2, the B3LYP results show a 2.5 bond order for the Cr-O bond, which includes one σ bond, one π bond, and a π bond with an α electron but no β electrons; this is also found in the orbital plots. The BP86 level gives a bond order of 2 with one σ bond and one π bond for the Cr-O in 3OCrF2. 24 ACS Paragon Plus Environment

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The difference is due to the larger spin polarization from the Mulliken spin density analysis (Supporting Information) on the O in 3OCrF2 as given by B3LYP than by BP86. Double bond character is predicted for the Mo-O and W-O bonds in 3OMoF2 and 3OWF2. The W-O in 1

OWF2 has triple bond character. The M-O σ bond for Cr-O in 3OCrF2 is essentially pure d on

the Cr at the B3LYP level but is predicted to have 20-25% s character at the BP86 level, with the difference again due to more spin polarization at the B3LYP level. For 3OMoF2, both functionals show a spin polarized σ bond with α component having up to 20% s and the β component being almost pure d in character. For 1OWF2, both functionals yield about 20% s character in the σ bond. For 3OCrF2 and 3OMoF2, the π bonds are almost pure d on the metal and one of the π on W in 1OWF2 also has essentially pure d character. The other π bond has ~65% d, ~21% p and ~ 13% f on the W. There is only one lone pair on the O for 1OWF2, so the second π orbital can be considered as a highly delocalize O lone pair. For the OMF compounds, the M-F single bonds are highly ionic. The terminal M-O bonds have triple bond character for all three metals. Even less metal density is found for the M-O bond in OMF than that in OMF2 so the M-O bonds in the former are even more ionic. For OCrF, the σ bond and a π bond have significant s character with remaining π bond having pure d character. A similar result is predicted for OMoF and OWF except that the π bond with s character has less s character than in OCrF. Conclusions The ternary group 6 oxide fluoride molecules, OMF2 and OMF, have been prepared and characterized by matrix isolation infrared spectroscopy and electronic structure calculations. Laser-ablated group 6 metal atoms reacted with OF2 to form OMF2 upon sample 25 ACS Paragon Plus Environment


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Page 26 of 35

deposition, the absorptions of which slightly increased when the sample was subjected to broad band irradiation and sample annealing. These complexes were identified by the M-O and F-M-F stretches with characteristic

18

O shifts when

18

OF2 was used as the reagent.

Triatomic OCrF, OMoF and OWF molecules appeared upon sample deposition as well. Identification of all three triatomic molecules was performed based on the M-O and M-F stretches, and for Mo, the resolved natural Mo isotopic splittings for a single Mo atom. For OCrF, the coupling between Cr-O and Cr-F stretches resulted in a low 1.0395 for the Cr-O stretch as well as an

18

16

O/ 18O ratio of

O shift of 4.5 cm-1 for the Cr-F stretch. Such

effect is negligible in OWF and much smaller in OMoF. The Cr-O bond distance of 1.569 Å in OCrF2 (Table 2) is shorter than the experimental value of 1.615 Å 57 and the calculated value of 1.613 Å 58 at the DK-CCSD(T)/CBS level in 5CrO. The calculated Mo-O and W-O bond distances in 3OMoF2 and 1OWF2 are also shorter than that in 5MoO and 5WO, which are predicted to be 1.707 and 1.714 Å at the B3LYP level with basis set used above, respectively. The longer M-O bond distances in the MO compounds are due to more electron-electron repulsion between the metal and O in MO than that in OMF2 arising from the removal of metal electron density by the F atoms. Similar differences are predicted for the M-F bond distances in OMF2 and OMF. There is more electron density on the metal in OMF than in OMF2 as the former has only one F atom to remove density from the metal center leading to more electron-electron repulsions. The character of the M-O triple bond in 1OWF2 resembles that in 1OHfF2 and the other group 4 OMF2 compounds18 which suggests the presence of a triple bond even though there are formally not enough electrons on the group 4 metals to generate such bonds if they 26 ACS Paragon Plus Environment

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were all covalent. These orbitals were previously described as a σ bond, a π bond, and a highly delocalized O lone pair forming the second π bond just as found for group 4 1OMF2 molecules. The 3OCrF2 and 3OMoF2 are best described as having a M-O double bond. We note that the formal oxidation state assignments would have the M-O and M-F bonds as being purely ionic with the metal being in the +IV oxidation state. At the other limit of pure covalency, there should be two M-F single bonds and a M-O double bond. The actual results are in between with highly polarized (ionic) M-F single bonds and M-O double bonds for 3

OCrF2 and 3OMoF2 and a M-O triple bond for 1OWF2.

Supporting Information. Complete citations for References 29 and 44. Figures: Molecular orbital diagrams for OMF2 and OMF, expanded scale infrared spectra of W + 18OF2 in excess argon. Tables: Calculated infrared absorptions and IR intensities, Mulliken spin densities, NPA charges and population analysis, and Orbital compositions. This material is available free of charge via the internet at http://pubs.acs.org. Acknowledgments This work was supported by the ‘‘Strategic Priority Research Program’’ and “Frontier Science Key Program” of the Chinese Academy of Sciences (Grant No. XDA02030000 and QYZDYSSW-JSC016) (X.C., Q.L., Y.G.), ‘‘Young Thousand Talented Program’’ (Y.G.), retirement funds from TIAA (L.A.). DAD acknowledges support from the U.S. Department of Energy (DOE), Office of Science (SC), Basic Energy Sciences (BES), catalysis center program. DAD thanks the Robert Ramsay Chair Fund of The University of Alabama for support. References 1

Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 27 ACS Paragon Plus Environment


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