ARTICLE pubs.acs.org/Organometallics
Infrared Spectra and Quantum Chemical Calculations of the Bridge-Bonded HC(F)LnF2 (Ln = La Lu) Complexes Yu Gong,† Xuefeng Wang,†,‡ Lester Andrews,*,† Mingyang Chen,§ and David A. Dixon*,§ †
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United States Department of Chemistry, Tongji University, Shanghai, 200092 China § Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama, United States ‡
bS Supporting Information ABSTRACT: Lanthanide metal atoms, produced by laser ablation, were condensed with CHF3 (CDF3) in excess argon or neon at 4 K, and new infrared absorptions are assigned to the oxidative addition product fluoromethylene lanthanide difluoride complex on the basis of deuterium substitution and density functional theory frequency calculations. Two dominant bands in the 500 cm 1 region are identified as metal fluorine stretching modes. A band in the mid-600 cm 1 region is diagnostic for the unusual fluorine bridge bond C (F) Ln. Our calculations show that most of the bridged HC(F)LnF2 structures are 3 6 kcal/mol lower in energy than the open CHF-LnF2 structures, which is in contrast to the open structures observed for the corresponding CH2-LnF2 methylene lanthanide difluorides. Argon-to-neon matrix shifts are 15 16 cm 1 to the blue for stretching of the almost purely ionic Ln F bonds, as expected, but 10 cm 1 to the red for the bridge C (F) Ln stretching mode, which arises because Ar binds more strongly to the electropositive Ln center, decreasing the bridge bonding, and thus allowing a higher C F stretching frequency.
’ INTRODUCTION Transition metal alkylidene complexes are important intermediates involved in a wide range of reactions. A large number of studies on the synthesis, characterization, and reactivity of these important species have been performed.1 4 However, the alkylidene complexes of the group 3 transition metals, and especially the lanthanide metals, have attracted less attention compared with the wide range of studies on other transition metal systems.5 Due to the limited number of valence electrons for the group 3 transition metals, typical alkylidene complexes with strong metal carbon bonds such as those of group 4 metals6 are not expected to be formed, so investigations on the structure and bonding of group 3 alkylidene complexes are of interest. Synthetic methods have contributed to this topic in recent years,7 10 as have studies of the simplest versions of such molecules using matrix isolation infrared spectroscopy.11 13 The insertion products and methylidene complexes of scandium, yttrium, and lanthanum generated by the reaction with methane and methyl halides have been prepared and characterized in noble gas matrixes.11 13 Our electronic structure calculations benchmarked by the infrared data show that the group 3 methylidene complexes basically have C M single bonds in contrast to the CdM double bonds found for the group 4, 5, and 6 and Th and U methylidene complexes.14 18 Recently the reactions of methyl fluoride and methylene fluoride with lanthanide atoms have been studied in our laboratories.19,20 The insertion and methylene products dominate the reactions of lanthanide atoms and CH3F,19 whereas only methylene complexes with an Ln C single bond were found in the CH2F2 reactions.20 r 2011 American Chemical Society
In the current work, we present new experimental and theoretical results on the reactions of lanthanide atoms with CHF3. Although the methylene complexes (CHFLnF2) are still the major reaction products, all are found to have bridged C F bonds, which differ from the open products observed in the reactions with CH2F2.
’ EXPERIMENTAL AND COMPUTATIONAL METHODS The experimental apparatus and procedure for investigating laserablated lanthanide metal atom reactions with CHF3 during condensation in excess argon and neon at 4 K have been described previously.21 The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a freshly cleaned lanthanide metal target (Johnson-Matthey) mounted on a rotating rod. Laser-ablated lanthanide metal atoms were co-deposited with 3 4 mmol of Ar (Matheson, research) containing 1% CHF3 or 1 1.5% CDF3 (prepared by reaction of CDCl3 with HgF2 at 150 C for 18 h) onto a CsI cryogenic window for 60 min. Complementary Ne matrix experiments were done with most metals. FTIR spectra were recorded at 0.5 cm 1 resolution on a Nicolet 750 FTIR instrument with 0.1 cm 1 accuracy using a HgCdTe range B detector. Matrix samples were annealed at different temperatures, and selected samples were subjected to broadband photolysis by a medium-pressure mercury arc street lamp (Philips, 175 W) with the outer globe removed. Density functional theory (DFT)22 calculations were performed using the Gaussian 03/09 program systems.23 The geometries of the Received: June 21, 2011 Published: July 18, 2011 4443
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Organometallics products of the Ln + CH3F reactions were optimized, and second derivatives were calculated to predict the vibrational frequencies for all lanthanide metals. The energies for the reaction CHF3 + Ln f CHFLnF2 for multiple spin states of CHFLnF2 were calculated to predict the ground spin state for each lanthanide reaction product molecule. The DFT calculations were performed with the B3LYP hybrid functional24 with the DZVP225 basis set on H, C, and F and the segmented small core relativistic effective core potential (ECP)26 from the Stuttgart group (ECP28MWB)27 with the corresponding segmented basis set (ECP28MWB_SEG)27 on the lanthanide elements.28 There are 28 electrons subsumed in the ECP leaving the 4s, 4p, 4d, 5s, 5p, 6s, and 4f (or 5d) electrons in the valence space for the Ln. The basis set is of the form (14s,13p,10d,8f,6 g/10s, 8p,5d,4f,2 g), and we denote this as the Stutt basis set. This combination of basis sets and exchange correlation functional has been found to work well for our analysis of similar reactions of Ln with CH2F2 and CH3F.19,20
’ RESULTS AND DISCUSSION Reactions of all of the lanthanide metal atoms (including La but not the radioactive Pm) and CHF3 were performed in an Ar matrix, and 11 of these were also studied in a Ne matrix. Absorptions due to CF3, CHF2, CF2, and other common bands produced from the photodecomposition of the CHF3 precursor were present in all of the experiments.29 32 Lanthanide trifluoride molecule products were observed in most of the matrix samples,33 while weak lanthanide difluoride absorptions were also found in the Yb, Sm, and Eu experiments.34 The infrared spectra from the reactions of different lanthanide atoms and CHF3 and CDF3 in Ar or Ne matrixes are illustrated in Figures 1 5, and the product absorptions are listed in Table 1. We consider several examples below. Infrared Spectra. Figure 1 compares the infrared spectra from the co-deposition of laser-ablated Yb atoms and CHF3 and CDF3 in solid Ar. The major product absorptions are 672.6, 562.6, and 549.5 cm 1 (labeled m for fluoromethylene lanthanide difluoride complex in Figure 1), which are barely observed upon sample deposition. Broadband irradiation increased these bands markedly, and they sharpened when the sample was further annealed. Complementary infrared spectra in a Ne matrix are shown in Figure 2, and these spectra exhibited a similar pattern for the product absorptions. New bands at 662.6, 578.0, and 564.9 cm 1 (labeled m in Figure 2) correspond to the above absorptions observed in the argon matrix. The 562.6 and 549.5 cm 1 absorptions lie in the region of Ln F vibrations. Experiments with CDF3 revealed very small red shifts of less than 0.5 cm 1, suggesting the hydrogen (deuterium) atom is involved very little in these two vibrational modes. We assign these two bands to the symmetric and antisymmetric F Yb F stretching vibrations; the results of the electronic structure calculations discussed below provide further support for this assignment. This identification of the Yb F vibrations is also supported by the large Ar-to-Ne blue shift (about 15 cm 1) due to the high ionic character of the Yb F bond. The 672.6 cm 1 absorption in solid Ar is diagnostic for the structural description of the product. This band exhibited a small deuterium shift of 2.7 cm 1, which is larger than that for the LnF2 vibrations mentioned above. As a result, a C F stretching vibration is proposed as the assignment for this band, although the band is significantly lower than the typical C F stretch in the 1000 to 1050 cm 1 range.35 Our DFT calculations (see below for details) show that the 672.6 cm 1 band position is indicative of a bridge C-(F)-Yb stretching vibration instead of a terminal C F stretch, which should be around 1000 cm 1 (compare the computed
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Figure 1. Infrared spectra of laser-ablated Yb atoms and fluoroform reaction products in solid argon at 4 K: (a) Yb + 1% CHF3 deposition for 60 min; (b) after λ >220 nm irradiation; (c) after annealing to 30 K; (d) Yb + 1% CDF3 deposition for 60 min; (e) after λ >220 nm irradiation; (f) after annealing to 30 K. The label m denotes fluoromethylene lanthanide difluoride complex. The asterisk denotes the CO2 absorption.
Figure 2. Infrared spectra of laser-ablated Yb atoms and fluoroform reaction products in solid neon at 4 K: (a) Yb + 0.5% CHF3 deposition for 40 min; (b) after λ >220 nm irradiation; (c) after annealing to 10 K; (d) Yb + 0.5% CDF3 deposition for 40 min; (e) after λ >220 nm irradiation; (f) after annealing to 10 K. The label m denotes fluoromethylene lanthanide difluoride complex. The asterisk denotes the CO2 absorption.
frequencies in Table 2 and Supporting Information). We note that the Ne matrix shifts for this band are 10 cm 1 to the red, in contrast to the above blue shifts for the ionic Yb F stretching modes. The deuterium shift remains the same 2.7 cm 1 lower for this C-(F)-Yb stretching mode in solid Ne. The observation of these three vibrational modes coupled with the computational results leads us to propose a CH(F)YbF2 molecule with a bridged C-(F)-Yb bond as the product of the reaction of Yb and CHF3. We observed the new band in the bridge C (F) Ln stretching mode region for six metals (Table 1). In the Lu case, following Yb, three new bands were observed at 648.0, 580.2, and 570.3 cm 1 in solid Ne and at 657.0, 565.4, and 554.5 cm 1 in solid Ar with 2.5 2.9 cm 1 deuterium shifts for the higher band and smaller deuterium shifts for the lower two bands (Figures S1, S2). In the Er case, like Yb, three bands were observed at 659.6, 570.6, and 558.1 cm 1 in solid Ne and 4444
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Figure 3. Infrared spectra of laser-ablated Ce atoms and fluoroform reaction products in solid argon at 4 K: (a) Ce + 1% CHF3 deposition for 60 min; (b) after λ >220 nm irradiation; (c) after annealing to 30 K; (d) Ce + 1% CDF3 deposition for 60 min; (e) after λ >220 nm irradiation; (f) after annealing to 30 K. The label m denotes fluoromethylene lanthanide difluoride complex.
Figure 4. Infrared spectra of laser-ablated lanthanide atoms and (1%) CHF3 reaction products in solid argon at 4 K. The arrows denote the fluoromethylene lanthanide difluoride complex absorptions. The asterisks denote absorptions of LnF3. All the infrared spectra were recorded after 10 min of λ >220 nm irradiation or after subsequent sample annealing. Broad bands common to fluoroform experiments labeled c (ref 32).
at 670.0, 555.7, and 542.4 cm 1 in solid Ar. In the Ho case, three analogous absorptions were observed in each matrix. Again the Ln F modes are blue-shifted by about 15 cm 1 higher in Ne, as is the case for the LnF3 molecules.33 The bridged C (F) Ln mode is red-shifted by about 10 cm 1 in Ne, with the same 2.4 to 2.1 cm 1 deuterium shift, and the latter mode behaves opposite the matrix shift expected for an ionic bond.36 The infrared spectra from co-deposition of laser-ablated Ce atoms and CHF3 are shown in Figure 3. A band centered around 492.1 cm 1 (labeled m in Figure 3) was observed upon deposition. This band increased on broadband photolysis and sharpened on annealing together with an increase in the intensity of the nearby CeF3 absorptions.33 The experiment with CDF3 revealed a 0.3 cm 1 red-shift for the 492.1 cm 1 absorption, suggesting a Ce F vibration for this band. Another weak band at 512.4 cm 1 was also observed in the CDF3 experiment. The
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Figure 5. Infrared spectra of laser-ablated La atoms and CHF3 reaction products in solid neon at 4 K: (a) La + 0.5% CHF3 deposition for 40 min; (b) after λ >220 nm irradiation; (c) after annealing to 10 K; (d) La + 0.5% CDF3 deposition for 40 min; (e) after λ >220 nm irradiation; (f) after annealing to 10 K. The label m denotes fluoromethylene lanthanide difluoride complex.
position of this band is expected to be about the same as the CHF3 precursor absorption, which results in it being masked by precursor in the CHF3 experiment. Thus, we assign the 512.4 cm 1 absorption observed in the CDF3 experiment to the symmetric F Ce F stretching vibrational mode, and the absorption at 492.1 cm 1 to the antisymmetric F Ce F vibration. No other band is found to track with the 492.1 cm 1 absorption in the terminal C F stretching region, which again suggests the presence of a bridge C (F) bond in the product structure. The absorption for a bridged C F stretching vibration is probably overlapped with the strong precursor absorption near 700 cm 1. Hence a fluorine bridge HC(F)CeF2 structure is proposed as the product from the reactions of Ce and CHF3. Figure 4 shows the infrared spectra in the region of Ln F stretching vibration from the reactions of all of the lanthanide atoms (except Pm) and CHF3 in solid Ar. The symmetric and antisymmetric F Ln F stretching vibrations were observed in this region for most metal products with the band positions blueshifted stepwise from La to Lu, which reflects the effect of lanthanide contraction.37 A similar blue shift has been found in the CH2LnF2 complexes as well.20 Pr is the only exception, with unusually higher frequencies than its neighbors. Both the symmetric and antisymmetric F Pr F stretching vibrational modes were blueshifted in the Ne matrix, while no new absorption was observed, which strongly supports the identification of these two bands for the Pr reaction product. The unusual frequencies in the Pr case are probably related to a vibronic perturbation with a low-lying electronic state. Such an explanation has been used to account for the difference in vibrational frequencies of PrF3 from those of the other lanthanide metal trifluorides.38 A similar anomaly was also observed in the Pr and CH2F2 reaction product frequencies.20 Only a single product absorption peak is observed in the spectra of La, Ce, and Nd, respectively (Figure 4). Following the trend for the symmetric and antisymmetric F Ln F vibrations, it is reasonable to assume that the missing bands for these three metals are overlapped by the CHF3 precursor absorption. In some cases, changing the matrix moves the precursor band and allows the observation of an additional product absorption. As shown in Figure 5, the infrared spectra from the reactions of laser-ablated La atoms and CHF3 in a Ne matrix clearly demonstrate the 4445
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Table 1. Infrared Absorptions of HC(F)LnF2 Molecules from the Reactions of Lanthanide Atoms and CHF3 in Solid Ar and Ne (frequencies in bold are for DC(F)LnF2) metal
bridging C F str. Ar
a
La
Pr 707.5 a
bridging C F str. Ne
sym. FLnF str. Ne
asym. FLnF str. Ne
483.8
521.0
499.9
520.4 527.7
499.5
512.4
491.8
526.8
505.5
543.3
518.7
560.4
532.9
543.2
518.5
523.9
a
538.0
521.2
a
523.3
Sm
533.1
512.4
546.6
527.7
Eu
532.8 534.9b
512.1 516.3b
546.6 550.1
527.3 532.3
534.4
515.9
Gd Tb
541.1b
523.0
541.0
522.9
546.4
531.4
547.6
561.2
546.3
531.3
547.3
560.9
Dy
674.8
549.6
539.4
Ho
671.7 671.4
549.1 552.0
539.0 539.7
661.3
567.0
555.6
669.0
551.9
539.7
659.2
566.9
555.6
Er
670.0 a
Tm
a
asym. FLnF str. Ar
483.6 492.1
a
Ce
Nd
sym. FLnF str. Ar
555.7
542.4
659.6
570.6
558.1
555.4
542.3
657.0
570.2
558.0
561.2
547.8
560.9
547.7
Yb
672.6
562.6
549.5
662.6
578.0
564.9
Lu
669.9 657.0
562.4 565.4
549.1 554.5
659.9 648.0
577.4 580.2
564.4 570.3
654.5
564.8b
554.4
645.1
580.2
570.3
Absorptions unresolved due to band overlap. b Major site at 20 K.
existence of two different F La F vibrations at 521.1 and 499.9 cm 1 (labeled m in Figure 5) with the CHF3 precursor absorption between these two bands. In the CDF3 reaction the precursor absorption moves 5.2 cm 1, but the product absorptions shift only 0.4 0.6 cm 1. The C F vibration assigned to the bridge fluorine was observed for most of the late HC(F)LnF2 complexes (Table 1), which strongly supports the assignment of the bridge fluorine structure. Although this mode was observed only for Nd among the early lanthanide products (Table 1), the bridge fluorine structure is still the most probable due to the lack of strong terminal C F vibrations in the 1000 1050 cm 1 region and the results of our electronic structure calculations (Tables 2 and 3). Electronic Structure Calculations. Our calculations show that the bridged structure HC(F)LnF2 (containing a C(F)Ln triangle) is the lowest energy structure for all Ln except for Gd, where the bridge structure was not predicted to form, and for Eu, where HC(F)EuF2 is ∼4 kcal/mol higher than the lowest energy double-bridged structure. In addition, it was not possible to optimize an open structure (no bridge bonds) for the molecules with La and Ce. The open structures are about 5 kcal/mol higher in energy for most of the molecules except for Ln = Pm and Eu, where the energy difference is close to 0 kcal/mol and for Tb with a 20 kcal/mol more stable bridged structure. We also optimized a double F-bridged structure for HC(FF)LnF. In all
cases, except for Eu (∼5 kcal/mol lower, see below) and Sm and Gd (∼10 kcal/mol higher), this structure is much higher in energy than the single F-bridge HC(F)LnF2 or open HFCLnF2 structures. If one of the bridged fluorine atoms transfers to C in the HC(FF)LnF structure, it forms HFC(F)LnF. All of the HFC(F)LnF have similar energies to the dibridged HC(FF)LnF. The energies for the reactions of CHF3 with Ln become less exothermic from La to Eu as the f orbitals on the Ln metal are occupied up to 7 f electrons. The reaction energy becomes more exothermic for Gd, with 7 f and 1 d electrons, and then increases substantially to Tb with an f9 configuration. As predicted for the early part of the series, as more f electrons are added, the reaction exothermicity decreases until Lu, with a filled f14 shell, where it increases substantially. F-Bridged Structures. The geometry of the bridge structure for HC(F)CeF2 is shown in Figure 6. The HC(F)LnF2 molecule with a bridging F center is an intermediate structure along the F migration pathway, which does not proceed to complete F transfer owing to the trivalence of the Ln metal. All of the methylene lanthanide fluoride complexes observed in the reactions of lanthanide metal atoms with CH3F and CH2F2 possess structures without bridge-bonded atoms,19,20 whereas structures with a bridged F were predicted in the CHF3 reactions for all Ln except Gd; for Eu, the structure HC(FF)LnF is lower in energy. In the HC(F)LnF2 molecules, one hydrogen and one fluorine atom are left on the carbon after the transfer of two fluorine 4446
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Table 2. Calculated Reaction and Isomerization Energies (kcal/mol) and Frequencies (cm 1) for Bridge HC(F)LnF2 and DC(F)LnF2 (in bold) Except Gd Ln La
spin 2
ΔErxna 156.5
Ce
3
154.0
Pr
4
119.9
Nd Pm
5 6
127.1 98.6
ΔEiso opena
ΔEiso CH(FF)LnFa
c
53.2
441 (107)
504 (243)
520 (104)
705 (114)
1210 (24)
51.6
417 (99) 450 (105)
504 (238) 514 (225)
520 (109) 536 (113)
701 (113) 726 (115)
911 (6) 1213 (24)
425 (97)
514 (222)
536 (118)
721 (114)
915 (7)
453 (101)
508 (237)
532 (107)
721 (118)
1212 (21)
428 (92)
509 (243)
533 (113)
718 (117)
915 (6)
c
4.3 4.7 0.3
42.7 38.8
1204 (20) 909 (5) 1229 (7)
420 (27)
522 (220)
529 (178)
738 (144)
930 (4)
381 (26)
513 (234)
521 (201)
906 (257)
1309 (7)
357 (21)
512 (214)
520 (204)
895 (226)
997 (34)
4.2
89.1
Tb
8
208.5
Dy Ho
7 6
157.9 126.3
7.6
c
c
19.6 3.1 5.1
48.2 32.0
532 (164)
549 (140)
1082 (390)
1307 (13)
336 (0) 402 (22)
532 (169) 529 (169)
548 (130) 548 (90)
1094 (359) 1118 (214)
973 (39) 1249 (64) 935 (16)
378 (24)
529 (170)
548 (85)
1124 (238)
470 (77)
553 (193)
564 (90)
685 (118)
1194 (21)
444 (73)
553 (191)
564 (93)
680 (117)
901 (5)
483 (74)
556 (186)
569 (89)
700 (120)
1193 (24)
455 (70)
555 (184)
568 (92)
695 (118)
903 (7)
661 (123)
1183 (21)
656 (121) 649 (121)
894 (6) 1180 (19)
451 (57)
559 (173)
570 (89)
644 (120)
890 (6)
477 (56)
561 (174)
574 (89)
647 (120)
1179 (21)
452 (56)
561 (172)
574 (89)
642 (118)
889 (6)
83.9
6.2
29.9
4.5
359 (1)
568 (81)
4
140.8
972 (29)
568 (83) 570 (88)
Tm
2
1305 (14)
1108 (307)
557 (186)
56.6
Lu
1097 (328)
550 (96)
557 (184) 559 (175)
5.6
5.4
551 (123)
529 (167)
480 (65)
104.3
64.4
529 (167)
337 (2)
454 (63) 476 (58)
5
3
363 (1)
79.1
Er
Yb
1207 (20) 911 (5)
712 (118)
0.3
9
718 (121) 713 (119) 707 (117) 744 (147)
56.9
Gd open
539 (102) 538 (108) 542 (96)
8
89.5
523 (233) 522 (230)
542 (101) 529 (178)
Eu
9
465 (101) 439 (92)
534 (238)
11.6
Gd open
F C H bend
534 (236) 522 (221)
2.2
c
C F strb
463 (97)
78.0
90.1
LnF2 sym
438 (89) 447 (28)
7
7
LnF2 asym
29.3
Sm
Gd open
Ln C str
11.8 50.3
479 (58)
566 (166)
577 (86)
638 (122)
1176 (20)
453 (57)
566 (164)
576 (164)
633 (121)
887 (6)
484 (52)
568 (161)
577 (79)
636 (124)
1171 (18)
457 (52)
568 (159)
577 (79)
630 (122)
884 (5)
a ΔErxn is the reaction energy of the CHF3 + Ln f HC(F)LnF2 reaction; ΔEiso is the isomerization energy for converting the single bridging HC(F)LnF2 to the open HFC-LnF2 and the double bridging HC(FF)-LnF. b C F (bridge) stretches for the HC(F)LnF2, and C F (open) stretches for HFC-LnF2. c No bridging structure found for Gd. No open structure found for La and Ce.
atoms to the metal center. For the reactions of CHF3 with Ln to form the +3 oxidation state, one F has to be bonded to the C, and these structures are not present when more hydrogen is present in the starting fluoromethane. In the current case, the CHF group can be considered as :CHF . Another way to write the structure would be as the Ln in the +2 oxidation state which forms a σ bond with the :CHF fluoromethylene species. In either case, the excess spin on the Ln is the same. The r(Ln C) bond distances are between 2.3 and 2.5 Å for most of the HC(F)LnF2 compounds, and the r(Ln F) bond distances are in the range 1.9 to 2.1 Å for the terminal F and 2.2 2.6 Å for the bridged F. The r(Ln C) and r(Ln F) bond distances generally decrease with increasing atomic number due to the lanthanide contraction.37 The values of r(C Fbridge) are approximately 1.5 Å and increase with increasing atomic number of the lanthanide as the F atom becomes more strongly bonded
to the Ln (Table 3). The r(C Fbridge) is substantially longer than the calculated C F bond length of 1.403 Å in CH3F and of 1.352 Å in CHF3. The r(Ln C) in HC(F)LnF2 are slightly shorter than the r(Ln C) in CH2LnF2 by up to 0.02 Å and slightly longer than those in CH2LnHF by up to 0.02 Å; so, there is very little change in r(Ln C) in all three compounds.19,20 The geometry of HC(F)LnF2 has the CH(F) group approximately orthogonal to the LnF2 plane. This is in contrast to the structures of CH2LnF2 and CH2LnHF, where the plane defining the CH2 groups is approximately parallel to the LnF2 plane. The structures with the CH2 groups have a low rotation barrier about the Ln C bond consistent with an electron distribution with an unpaired electron on the C. The electron distribution is essentially the same in HC(F)LnF2 for most of the lanthanides. As shown in Table 3 and Figure 6 (except for Gd, where no single bridge structure is predicted and Eu with only 0.6 e on the C), the 4447
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Table 3. Geometry Parameters and Mulliken Charges for Bridged Speciesa metal r(Ln C) r(C F) r(Ln F) bridge r(Ln F) — CFLn bridge Ln charge F bridge charge F(Ln) charge CH group charge spin on C spin on Ln
a
La
2.479
1.514
2.507
2.117
71.3
1.416
0.308
0.485
0.137
0.98
0.02
Ce
2.440
1.503
2.512
2.092
69.7
1.398
0.306
0.481
0.130
0.96
1.04
Pr
2.420
1.505
2.479
2.081
69.9
1.415
0.307
0.487
0.134
0.95
2.07
Nd
2.394
1.507
2.460
2.072
69.6
1.429
0.309
0.496
0.128
0.94
3.07
Pm
2.386
1.511
2.430
2.066
70.1
1.452
0.311
0.505
0.131
0.95
4.07
Sm
2.388
1.487
2.456
2.063
69.7
1.440
0.293
0.519
0.108
0.85
5.17
Eu
2.460
1.419
2.652
2.076
66.6
1.377
0.231
0.552
0.041
0.58
6.45
Tb Dy
2.344 2.317
1.527 1.519
2.353 2.364
2.015 2.005
70.7 69.4
1.450 1.435
0.316 0.314
0.502 0.495
0.130 0.131
0.98 1.00
6.01 4.97
Ho
2.311
1.543
2.300
2.006
70.8
1.474
0.294
0.513
0.122
0.99
4.00
Er
2.308
1.552
2.266
1.997
71.6
1.474
0.323
0.513
0.125
1.00
3.00
Tm
2.299
1.553
2.249
1.990
71.8
1.471
0.320
0.514
0.122
1.00
1.99
Yb
2.287
1.560
2.246
1.976
71.3
1.477
0.296
0.512
0.122
1.00
0.99
Lu
2.272
1.564
2.223
1.970
71.3
1.528
0.338
0.534
0.122
1.01
0.01
No bridged Gd species could be optimized.
Figure 6. Molecular orbital diagrams (isovalue = 0.06 e au 3) of (a) unpaired C 2p, (b) unpaired Ce 4f, (c) Ce C bond (ionic), and (d g) C F, C H, and F lone pairs and (h) spin density diagram (isovalue = 0.005 e au 3for HC(F)CeF2).
C in HC(F)LnF2 has essentially a single electron in a p orbital perpendicular to the Ln C bond and the remaining unpaired spin density on the Ln (see the spin density for HC(F)CeF2, Figure 6h). The rotation of the p orbital by 90 enables the F to bridge to the Ln and minimizes any steric interactions with the F atoms on the Ln. Similar total spin densities are obtained for the open CHFLnF2, CH2LnF2, and CH2LnHF molecules. These results suggest that the electronic structures of the bridgebonded HC(F)LnF2 molecules are similar to those of the open structure for CHFLnF2, especially for the Ln C bonding. The C Fbridge bond has substantial p character (Figure 6). The orbitals in Figure 6d and f show the “bent-sigma”-like interactions between the F 2p and C 2p lone pairs. Figure 6e depicts delocalization of a lone pair on F to from a partial π bond with the C. The orbitals clearly show that the interaction of the bridge F with the Ln is mostly ionic in nature. The bridge C (F) bond interaction with a metal center is unusual, as numerous examples of transition metal compounds X2C(Cl)MX with a bridging chlorine atom have been observed, but only one with a bridging fluorine, H2C(F)NiCl.39,40 For this latter molecule,
involving a first-row transition metal atom, the C F bond is shorter and the frequency for the bridge fluorine is higher (845 cm 1), showing a smaller bridging interaction than found in the lanthanide molecules under study. Although the single C Ln bond is longer than that in the Ni case, the bonding interaction between the bridge F and the C is larger in the Ni compound (and weaker with the Ni) than in the lanthanides, consistent with the lower C (F) stretching vibrational frequency as well as the smaller (F) C Ln bond angle in the lanthanides. This stronger interaction of the F with the lanthanide could arise due to the larger radius of the Ln. The HC(F)LnF2 bridge bond arises from interactions of the negative bridge-F atom with the positive charge on the Ln; so the Ln Fbridge bonding will be dominated by the electrophilicity (charge) of the metal center. If the metal is more electrondeficient (positive charge), then there will be more transfer of the F toward the metal and less interaction with the carbon. It is not easy to form an H-bridged species, as the hydrogen has an electron affinity of only 0.75 eV and hence will be much less negative as well as much smaller. Chlorine has a higher electron 4448
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Organometallics affinity than F, is larger, and its electron density is more easily polarized; so we would expect compounds with Cl bridges to be more common than compounds with F bridges. The Ln F bonds in HC(F)LnF2 are highly ionic (Figure 6), just as found for CH2LnF2. The orbitals show that the C Ln bond is highly ionic as well (Figure 6). In CH2LnF2, there is no π bond between the Ln and C. Instead, there is a lone electron on the C in a p orbital approximately perpendicular to the C Ln axis,20 so that CH2LnF2 is best described as a metal-substituted methyl radical. The Ln C bond in CH2LnF2 can be described as arising from the electron distribution of the triplet ground state of the carbene CH2 with one electron in the CH2 plane and one electron perpendicular to the plane. The in-plane electron forms the Ln C bond. However, the ground state of CHF is a singlet with both electrons nominally in the CHF plane, and the excited state is the triplet calculated to be 11.0 kcal/mol higher in energy. Thus, the Ln reaction energies for addition of CHF3 to form CH(F)LnF2 should be less negative than the reaction energies for CH2F2 and CH3F with Ln, as found. The first C F bond energy in CHF3 is stronger than that in CF2H2 or CH3F, again consistent with the lower reaction energies for the former.41 The difference in the ability of the ground state of CH2 and CHF to bind to the Ln may also influence the formation of the bridged C (F) Ln bond. The fact that CHF has to be excited to the triplet state to generate a better Ln C bond means that the bond is inherently less stable. The F in the triplet state is slightly more negative by 0.03 e than is the F in the singlet state. In addition, the HCF bond angle opens up from 101.5 in the singlet to 121.1 in the triplet. In order to further stabilize the Ln C interaction, the negative F can bond with the cationic Ln. This is enabled by the increase in the HCF bond angle and the enhancement of the negative charge on the F bonded to C. For Ln = La and Ce, we attempted to optimize the open HFCLnF2 structures, but the optimization always resulted in the bridged HC(F)LnF2. The energy difference between the open and the bridged structures is ∼5 kcal/mol for Ln = Pr and Nd and decreases to less than 2 kcal/mol for Ln = Pm, Sm, and Eu for the early lanthanides. Gd is the only Ln for which we could not optimize a bridged HC(F)LnF2 structure, which is consistent with the decrease in the isomerization energy with increasing atomic number for the early lanthanides. We predict the septet state to be the lowest energy for HFC-GdF2, with two nonet states less than 1 kcal/mol higher in energy. These three molecules have similar geometries and have similar predicted spectra especially for the Gd F antisymmetric and symmetric stretching modes in the GdF2 moiety. The open/bridge energy differences increases to 20 kcal/mol for Ln = Tb, which is the largest energy difference excluding La and Ce. The energy differences then decrease to 3 kcal/mol for Ln = Dy and are ∼5 kcal/mol for the remaining lanthanides. The energy differences between the dibridged HC(FF)LnF and the monobridge HC(F)LnF2 decrease from 52 kcal/mol for Ce to 12 kcal/mol for Sm and then become negative for Eu. Our calculations show for Ln = Eu that the open insertion product converges to the dibridged product. Although there is a small imaginary frequency of ∼40i cm 1, the dibridged structure is the lowest energy structure, 61 kcal/mol more stable than the reactants. However the predicted vibrational frequencies of the weak Eu C stretching mode at 460 cm 1, the strong Eu Fterminal stretching mode at ∼470 cm 1, and two strong symmetric and antisymmetric C F stretching mode at ∼700 and ∼830 cm 1 are not observed. The HFC(F)EuF complex is 1 kcal/mol less
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stable than the dibridged structure. It is predicted to have a strong Ln F stretching mode near 480 cm 1 and a strong C Fbridge stretching mode near 600 cm 1, which are also not observed. The C Fterminal stretching mode in HFC(F)EuF is calculated to be at 1044 cm 1 with a strong intensity and H C F bend ing mode at 1188 cm 1 with a moderate intensity. Although bridged HC(F)EuF2 is predicted to be ∼4 kcal/mol less stable than HC(FF)EuF, the calculated vibrational frequencies for HC(F)EuF2 are in good agreement with the experimental IR results. The prediction for the antisymmetric EuF stretching mode is within 5 cm 1 of experiment and within 10 cm 1 of experiment for the predicted symmetric EuF stretching mode. We thus assign HC(F)EuF2 as the major product for the Eu + CHF3 reaction. As the calculations are done at the DFT level, errors in the relative energies of several kcal/mol are possible. Eu in the +2 oxidation state has an f7 configuration and in the +3 has an f6 configuration. The complete f7 shell is stable so the Ln tries to reach this configuration by borrowing spin from the C. Thus, the best way to consider the structure of HFC(F)EuF may be as a sum of the following structures {[C(p1) + Ln (f6)] + [C(p0) + Ln (f7)]}. The Mulliken charge analysis shows that HC(F)EuF2 has the lowest positive charge on the Ln for all of the HC(F)LnF2, consistent with this argument. The spin distribution of 0.58e on C and 6.45e on Eu is also consistent. Matrix Effects. The unusual 10 cm 1 red shift in the bridged C (F) Ln stretching mode from the solid Ar to the Ne environment requires an explanation, as the usual effect of medium polarizability is a blue matrix shift like that found for the strong polar Ln F bond stretching vibrations.33,34 The strongest matrix interaction for the HC(F)LnF2 molecules will be at the electropositive metal center; for example, the Mulliken charge on Yb is +1.48e (Table 3). In order to estimate the effect of the rare gas, we predicted the structures and vibrational spectra for simple model complexes with one or two Ar or Ne atoms interacting with the Yb center. The resulting frequencies for six model complexes are given in Table 4, and the structures are illustrated in Figure 7. The effect of binding Ar and Ne atoms to the Yb center on the bridge C F stretching mode is to weaken the interaction of the Ln with bridging F and strengthen [less red shift in the C F mode] the C F bonding interaction, resulting in a blue shift in the bridge C F stretching mode. The syn interaction of the rare gas on the side where the C F Yb bridge is present is larger than if the rare gas is anti to the bridged F. Argon is a more polarizable rare gas than neon and has a stronger interaction with Yb, so Ar has a larger effect on the C F bridge bond. This leads to a larger blue shift in the C F frequency relative to the isolated molecule. The Ne atom, as expected, has a smaller interaction with the Yb center than does Ar, and hence its blue shift relative to the isolated molecule is smaller than that for Ar. This leads to the experimental observation of a red shift in the C F Yb bridge stretching mode when the matrix is changed from Ar to Ne. The rare gases are effectively removing positive charge from the metal, which weakens the interaction with the bridging F atom. Reaction Pathways in the Matrix. On the basis of considerable experience with laser-ablated metal atom reactions,6,11 18,40 the first reaction is insertion into a C F bond to form the product HF2CLnF followed by F-transfer rearrangement to form the observed HC(F)LnF2 species. As an example, consider Ln = Lu with all reaction energies given with respect to the separated reactants, Lu + CHF3. The direct insertion step to form HC(FF)LuF similar in structure to HF2CLuF but with two F atoms 4449
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Table 4. Calculated Reaction Energies (kcal/mol), Bond Lengths (Å), and Frequencies (cm 1) for Bridged HC(F)YbF2 and Model Ar and Ne Complexesa HC(F)YbF2e
property b
E B3LYP
A(Ar)
B(Ar)
C(Ar2)
D(Ne)
E(Ne)
F(Ne2)
1.4
1.2
1.6
0.8
0.7
2.7 1.559
5.9 1.550
1.4 1.555
1.4 1.561
3.2 1.557 2.245
1.2
E CCSD(T)c r(C F) A
1.560 [1.387]
2.8 1.550
r(F–Yb) A
2.246 [3.232]
2.269
2.228
2.262
2.257
2.227
r(C Yb) A
2.287 [2.312]
2.287
2.304
2.294
2.287
2.301
2.293
479 (58)
480 (63)
467 (47)
473 (57)
480 (59)
470 (49)
475 (56)
Yb C str cm
1
(km/mol) 1
YbF2 asym str cm YbF2 sym str cm
1
(km/mol)
(km/mol)
C F bridge strd cm F C H bend cm Mulliken on Yb
1 1
(km/mol)
(km/mol)
566 (166)
563 (157)
561 (157)
559 (152)
564 (162)
565 (164)
545 (187)
577 (86)
574 (81)
574 (80)
568 (77)
577 (86)
576 (83)
575 (67)
638 (122)
654 (122)
641 (124)
652 (125)
645 (122)
639 (121)
646 (124)
1176 (20) 1.477 [1.410]
1183 (19) 1.438
1179 (19) 1.429
1185 (18) 1.411
1180 (19) 1.440
1178 (20) 1.431
1181 (17) 1.403
a
E = E(complex) E(HC(F)YbF2) nE(Ar/Ne), n = 1, 2. b Noble (Ar and Ne) gas binding energies in kcal/mol using B3LYP/DZVP2+Stuttgart +aug-cc-pVDZ. Negative energy means binding. c Noble (Ar and Ne) gas binding energies in kcal/mol using U/UCCSD(T)/DZVP2+Stuttgart+aug-ccpVDZ with zero-point energies from the previous DFT calculations. Negative energy means binding. d Observed C-F str 672.6 cm 1 in solid argon and 662.6 cm 1 in solid neon. e Values for open HFC-YbF2 structure bond distance and Mulliken charge in brackets.
Figure 7. Calculated structures of the HC(F)YbF2(Ng)x (Ng = Ar, Ne, x = 1, 2) complexes.
on C leaning toward Lu is 90 kcal/mol exothermic, the reaction to form the bridge HFC(F)LuF complex is 93 kcal/mol exothermic, the reaction to produce the open HFCLuF2 complex is 136 kcal/mol exothermic, and the reaction to form the final observed bridge structure HC(F)LuF2 is 141 kcal/mol exothermic. The high exothermicity for the final product suggests that the HC(FF)LuF insertion intermediate and the F transfer HFC(F)LuF intermediate are not expected to be trapped in solid matrixes. HC(FF)EuF is the exception. Unlike the group 4, 5, and 6 transition metal cases,6,14 16 the three-F-transfer product with a C Ln triple bond is not produced for the lanthanide metals due in part to the inability of the Ln metal to form π bonds with the available electrons on the C, as already shown for CH2LnF2.20 Although the +4 oxidation state is expected to be stable for some lanthanide atoms such as Ce, our previous studies revealed that
Ce behaved similarly with other lanthanide metal atoms when reacted with CH2F2.20 The single electron on the Ce center remained unpaired in the H2C-CeF2 complex, resulting in a single C Ce bond. In our current experiments with CHF3, no absorption can be assigned to the HC-CeF3 molecule, which suggests that all of the lanthanide atoms have the same reaction mechanism toward CHF3 and that the +3 oxidation state dominates in all of the reaction products. Note that the decomposition products of CHF3 are also present in the matrix, indicating the production of fluorine atoms during sample deposition. These fluorine atoms can diffuse and react with lanthanide atoms on sample annealing, which accounts for the observation of LnF3 and some LnF2 molecules in our experiments. If a transient HC-CeF3 intermediate were formed, it decomposes straightaway to the HC and CeF3 species. 4450
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Organometallics
’ CONCLUSIONS The reactions of lanthanide atoms and CHF3 were studied using matrix isolation infrared spectroscopy and theoretical calculations. The major product for all of the lanthanide atoms is the fluoromethylene lanthanide difluoride complex, HC(F)LnF2, with an unusual bridged fluorine. The reaction product is proposed to be formed via insertion of the Ln into a C F bond followed by the transfer of a second fluorine. In most cases, the DFT calculations predict that the bridged HC(F)LnF2 structures are slightly more stable than the open HFCLnF2 isomers. The unusual blue shift of the bridged C (F) stretching frequency from neon to argon arises from the stronger interaction between argon atom(s) and the metal center of the lanthanide difluoride complex, which weakens the Ln (F) bridge-bonding interaction and allows the C (F) stretching frequency to increase. The formation of the bridge bonded F atom is consistent with the need for the CHF fragment to be in the triplet state to bind to the Ln. An energy on the order of 11 kcal/mol (calculated) is required to excite CHF from the ground-state singlet. The formation of the triplet CHF increases the HCF bond angle and the negative charge on the F so that the F can interact with the Ln to further stabilize the Ln C interaction. The participation of the triplet CHF fragment is consistent with the presence of an unpaired electron on the C. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figure S1: infrared spectra of laser-ablated Lu atoms and CHF3 reaction products in solid argon at 4 K; Figure S2: infrared spectra of laser-ablated Lu atoms and CHF3 reaction products in solid neon at 4 K; Table S1: calculated reaction and isomerization energies and frequencies for open HFCLnF2 and DFCLnF2 except for Ln = La and Ce; Table S2: calculated reaction and isomerization energies and frequencies for the dibridged HC(FF)-LnF; Table S3: optimized Cartesian x, y, z coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected],
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge financial support from DOE Office of Science, Basic Energy Sciences Grant No. DE-SC0001034 and NCSA computing Grant No. CHE07-0004N (University of Virginia) and Grant No. DE-FG02-03ER15481 (University of Alabama). D.A.D. thanks the Robert Ramsay Fund at the University of Alabama for partial support. J. Thrasher (University of Alabama) kindly provided the CDF3 sample. ’ REFERENCES (1) (a) Schrock, R. R. Chem. Rev. 2002, 102, 145. (b) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (c) Schrock, R. R. Chem. Commun. 2005, 2773. (2) (a) Scott, J.; Mindiola, D. J. Dalton Trans. 2009, 38, 2387. (b) Mindiola, D. J.; Scott, J. Nat. Chem. 2011, 3, 15. (3) (a) Herndon, J. W. Coord. Chem. Rev. 2006, 250, 1889. (b) Harder, S. Coord. Chem. Rev. 2011, 255, 1252.
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(4) de Fremonta, P.; Marionb, N.; Nolanb, S. P. Coord. Chem. Rev. 2009, 253, 862. (5) Giesbrecht, G. R.; Gordon, J. C. Dalton Trans. 2004, 33, 2387. (6) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040. (7) (a) Mills, D. P.; Soutar, L.; Lewis, W.; Blake, A. J.; Liddle, S. T. J. Am. Chem. Soc. 2010, 132, 14379. (b) Mills, D. P.; Cooper, O. J.; McMaster, J.; Lewis, W.; Liddle, S. T. Dalton Trans. 2009, 4547. (c) Liddle, S. T.; McMaster, J.; Green, J. C.; Arnold, P. L. Chem. Commun. 2008, 1747. (8) Zimmermann, M.; Rauschmaier, D.; Eichele, K.; Toernroos, K. W.; Anwander, R. Chem. Commun. 2010, 46, 5346. (9) Scott, J.; Fan, H.; Wicker, B. F.; Fout, A. R.; Baik, M. H.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 14438. (10) (a) Fustier, M.; Le Goff, X. F.; Le Floch, P.; Mezailles, N. J. Am. Chem. Soc. 2010, 132, 13108. (b) Cantat, T.; Mezailles, N.; Auffrant, A.; Le Floch, P. Dalton Trans. 2008, 37, 1957. (c) Cantat, T.; Jaroschik, F.; Nief, F.; Ricard, L.; Mezailles, N.; Le Floch, P. Chem. Commun. 2005, 5178. (d) Cantat, T.; Jaroschik, F.; Ricard, L.; Le Floch, P.; Nief, F.; Mezailles, N. Organometallics 2006, 25, 1329. (e) Buchard, A.; Auffrant, A.; Ricard, L.; Le Goff, X. F.; Platel, R. H.; Williams, C. K.; Le Floch, P. Dalton Trans. 2009, 10219. (f) Dietrich, H. M.; Toernroos, K. W.; Anwander, R. J. Am. Chem. Soc. 2006, 128, 9298. (11) Cho, H. G.; Andrews, L. Organometallics 2007, 26, 633. (12) Cho, H. G.; Andrews, L. J. Phys. Chem. A 2007, 111, 2480. (13) Cho, H. G.; Andrews, L. Inorg. Chim. Acta 2008, 361, 551. (14) (a) Lyon, J. T.; Andrews, L. Inorg. Chem. 2007, 46, 4799. (b) Lyon, J. T.; Andrews, L. Organometallics 2006, 25, 1341. (c) Cho, H. G.; Andrews, L. Inorg. Chem. 2005, 44, 979. (d) Cho, H. G.; Andrews, L. Organometallics 2004, 23, 4357. (e) Cho, H. G.; Andrews, L. J. Am. Chem. Soc. 2004, 126, 10485. (f) Cho, H. G.; Andrews, L. Inorg. Chem. 2004, 43, 5253. (g) Cho, H. G.; Andrews, L. J. Phys. Chem. A 2004, 108, 6294. (15) (a) Cho, H. G.; Andrews, L. J. Phys. Chem. A 2006, 110, 10063. (b) Cho, H. G.; Andrews, L. Organometallics 2006, 25, 477. (16) (a) Lyon, J. T.; Cho, H. G.; Andrews, L. Organometallics 2007, 26, 6373. (b) Cho, H. G.; Andrews, L. J. Phys. Chem. A 2006, 110, 13151. (c) Cho, H. G.; Andrews, L. Organometallics 2005, 24, 5678. (d) Cho, H. G.; Andrews, L. Chem.—Eur. J. 2005, 11, 5017. (17) (a) Lyon, J. T.; Andrews, L. Eur. J. Inorg. Chem. 2008, 1047. (b) Andrews, L.; Cho, H.-G. J. Phys. Chem. A 2005, 109, 6796. (c) Lyon, J. T.; Andrews, L. Inorg. Chem. 2005, 44, 8610. (18) (a) Lyon, J. T.; Andrews, L.; Hu, H.-S.; Li, J. Inorg. Chem. 2008, 47, 1435. (b) Lyon, J. T.; Andrews, L.; Malmqvist, P. A.; Roos, B. O.; Wang, T.; Bursten, B. E. Inorg. Chem. 2007, 46, 4917. (c) Lyon, J. T.; Andrews, L. Inorg. Chem. 2006, 45, 1847. (19) Chen, M. Y.; Dixon, D. A.; Wang, X. F.; Cho, H. G.; Andrews, L. J. Phys. Chem. A 2011, 115, 5609 (Ln+CH3F). (20) Wang, X. F.; Cho, H. G.; Andrews, L.; Chen, M. Y.; Dixon, D. A.; Hu, H. S.; Li, J. J. Phys. Chem. A 2011, 115, 1913 (Ln+CH2F2). (21) (a) Andrews, L.; Citra, A. Chem. Rev. 2002, 102, 885. (b) Andrews, L. Chem. Soc. Rev. 2004, 33, 123. (22) (a) Hohenberg, P.; Kohn, W. Phys. Rev. 1964, 136, B864. (b) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, A1133. (c) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford Univ. Press: Oxford, 1989. (d) Salahub, E. D. R.; Zerner, M. C. The Challenge of d and f Electrons; ACS: Washington, D.C., 1989. (23) 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, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; 4451
dx.doi.org/10.1021/om200533q |Organometallics 2011, 30, 4443–4452
Organometallics
ARTICLE
Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, € Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; S.; Daniels, A. D.; Farkas, O.; Fox, D. J. Gaussian 09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. Frisch, M. J.; et al. Gaussian 09, Revision A.2; Gaussian, Inc.: Wallingford, CT, 2009. (24) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, Y.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (25) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem. 1992, 70, 560. (26) (a) Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1989, 90, 1730. (b) Cao, X.; Dolg, M. J. Chem. Phys. 2001, 115, 7348. (27) http://www.theochem.uni-stuttgart.de/pseudopotentials/index. en.html. (28) Cao, X.; Dolg, M. J. Molec. Struct. (THEOCHEM) 2002, 581, 139. (29) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1968, 48, 2265. (30) Forney, D.; Jacox, M. E.; Irikura, K. K. J. Chem. Phys. 1994, 101, 8290. (31) (a) Carver, T. G.; Andrews, L. J. Chem. Phys. 1969, 50, 5100. (b) Jacox, M. E. J. Mol. Spectrosc. 1980, 81, 349. (32) Andrews, L.; Prochaska, F. T. J. Phys. Chem. 1979, 83, 824. (33) Kovacsa, A.; Konings, R. J. M. J. Phys. Chem. Ref. Data 2004, 33, 337, and references therein. (34) DeKock, C. W.; Wesley, R. D.; Radtke, D. D. High Temp. Sci. 1972, 4, 41. (35) Shimanouchi, T. Tables of Molecular Vibrational Frequencies Consolidated; National Bureau of Standards: Washington, D.C., 1972; Vol. I. (36) Jacox, M. E. Chem. Phys. 1994, 189, 149. (37) Greenwood, N, N,; Earnshaw, A. Chemistry of the Elemenets; Pergamon Press: Oxford, 1984; pp 1429 1432. (38) Lesiecki, M.; Nibler, J. W.; DeKock, C. W. J. Chem. Phys. 1972, 57, 13852. (39) (a) Cho, H.-G.; Andrews, L.; Vlaisavljevich, B; Gagliardi, L. Organometallics 2009, 28, 6871. (b) Cho, H.-G.; Lyon, J. T.; Andrews, L. Organometallics 2008, 27, 5241. (40) Cho, H.-G.; Andrews, L. Organometallics 2009, 28, 5623. (41) Luo, Y.-R. Comprehensive Handbook of Chemical Bond Energies; CRC Press, Taylor and Francis Group: Boca Raton, FL, USA, 2007.
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