Infrared Spectra of CX3−MnX and CX2 MnX2 (X = H, F, Cl) Prepared

Han-Gook Cho and Lester Andrews*. Department of Chemistry, University of Incheon, 177 Dohwa-dong, Nam-ku, Incheon, 402-749, South Korea, and ...
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Organometallics 2011, 30, 477–486 DOI: 10.1021/om100791h

477

Infrared Spectra of CX3-MnX and CX2dMnX2 (X = H, F, Cl) Prepared in Reactions of Laser-Ablated Manganese Atoms with Halomethanes Han-Gook Cho and Lester Andrews* Department of Chemistry, University of Incheon, 177 Dohwa-dong, Nam-ku, Incheon, 402-749, South Korea, and Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States Received August 13, 2010

Small manganese insertion (CX3-MnX) and methylidene (CX2dMnX2) complexes, carrying the highest multiplicities (sextet and quartet) among the analogous group 3-12 metal complexes, are produced in laser-ablated Mn atom reactions with halomethanes and identified in matrix infrared spectra with isotopic shifts and DFT frequency calculations. The linear C-M-X structure of the Mn insertion complexes resembles that of Grignard reagent molecules unlike those of other transition-metal analogues except Fe, and the Mn-C bond bears high s character, on the basis of DFT calculations. The Mn methylidenes have planar structures, common among early transition-metal analogues, revealing that Mn has borderline properties between the early and late transition metals. The computed C-Mn bond lengths of the carbene complexes in the quartet states (1.855-1.872 A˚) are considerably shorter than those of the insertion complexes in the sextet states (2.057-2.120 A˚), which is likely due to the one-half π-bond order and large ionic contribution to bonding in the carbene complexes. The tendency of increasing preference for higher oxidation-state products on going down in a family group is most dramatic among the group 7 metals Mn and Re.

Introduction Manganese is an essential element in daily food consumption and has important industrial uses, such as in steel production for strength, hardness, and durability.1 Manganese nodules, produced from the hot volcanic waters, are found in large quantity on the deep ocean floor and provide an important future natural resource.2 This group 7 metal is cheap and toxicologically benign, and it is the 12th most abundant element in the Earth’s crust, the next transition metal after iron.3 Organomanganese reagents are generally more stable than most organometallics from other transition metals, which is a favorable property for manganese-mediated reactions and catalytic activities.4 Since synthesis of the first organomanganese complex in 1937 to form phenylmanganese iodide from phenyllithium and manganese(II) iodide, reactions of manganese halides with orgnanolithium or organomagnesium compounds (transmetalation) still remain a primary route to provide these first-row transition-metal compounds.3 Manganese carbonyl derivatives such as [BrMn(CO)5] and [LiMn(CO)5] are also precursors to generate alkyl, aryl, and acyl products. The reactivities of organomanganese compounds are often compared to those of organomagnesium and organozinc compounds. Note that the

Mn electronegativity (1.55) is similar to those for Mg and Zn (1.55 and 1.65).5 Several simple neutral manganese-bearing species that are relevant to the present work have been studied. The reaction of atomic manganese with CH4 or H2O yields CH3-MnH or HMnOH following photoexcitation, but thermal Mn atoms react spontaneously with CH2N2 to form MnCH2 in matrix isolation investigations.6 Dimethyl manganese, methyl manganese chloride, and other alkyl manganese species have been synthesized by metathesis of Grignard reagents with manganese chloride, but they decompose rapidly.7 The first examples of manganocene-carbene complexes have been prepared by reaction of the sterically demanding nucleophilic carbenes 1,3-bis(2,6-dimethyl-4-bromophenyl)imidazol-2ylidene and 1,3-dimesitylimidazol-2-ylidene with manganocene.8a

*Author to whom correspondence should be addressed. E-mail: [email protected]. (1) http://www.webelements.com/manganese. (2) Cronan, D. S. In Encyclopedia of Ocean Sciences; Steele, J.; Turekian, K.; Thorpe, S., Eds.; Academic Press: San Diego, 2001; pp 1526-1533. (3) Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; WileyVCH: Weinheim, Germany, 2003; Vol. 20, p 497. (4) Cahiez, G.; Duplais, C.; Buendia, J. Chem. Rev. 2009, 109, 1434, and references therein.

(5) Comprehensive Organometallic Chemistry III; Mingos, M. P., Crabtree, R. H., Eds.; Elservier Ltd.: Oxford, U.K., 2007; Vols. 10 and 11. (6) (a) Billups, W. E.; Konarski, M. M.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1980, 102, 7394 Mn þ CH4. (b) Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. J. Phys. Chem. 1985, 89, 3541 Mn þ H2O. (c) Billups, W. E.; Chang, S.-C.; Margrave, J. L.; Hauge, R. H. Organometallics 1999, 18, 3551 Mn þ CH2N2. (7) (a) Riemschneider, R.; Kassahn, H. G.; Schneider, W. Z. Naturforsch. B 1960, 15, 547. (b) Beermann, C.; Clauss, K. Angew. Chem. 1959, 71, 627. (c) Tamura, M.; Kochi, J. J. Organomet. Chem. 1971, 29, 111 Mn alkyl complexes. (8) (a) Abernethy, C. D.; Cowley, A. H.; Jones, R. A.; Macdonald, C. L. B.; Shukla, P.; Thompson, L. K. Organometallics 2001, 20, 3629. (b) Braunschweig, H.; Burzler, M.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 8071. (c) Ruiz, J.; Perandones, B. F.; Garc'a, G.; Mosquera, M. E. G. Organometallics 2007, 26, 5687. (d) Swope, W. C.; Schaefer, H. F., III. Mol. Phys. 1977, 34, 1037. Xiang, H.; Yang, J.; Hou, J. G.; Zhu, Q. J. Am. Chem. Soc. 2006, 128, 2310. Pandey, R.; Rao, B. K.; Jena, P.; Blanco, M. A. J. Am. Chem. Soc. 2001, 123, 3799 Mn π-complexes.

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This complex featured a C(1)-Mn(1) bond length of 2.227 A˚ from an X-ray crystal structure. Related derivative complexes provided similar C-Mn bond lengths. The Mn carbene complex [(η5-C5H5)(OC)2MndCPh2] has been formed through borylene metathesis reactions.8b N-Heterocyclic carbene complexes of Mn(I) have also been synthesized with a shorter Mn-C bond length of 2.068 A˚.8c Simple manganese π-complexes with C2H2, C2H4, and C6H6 have also been investigated theoretically.8d Recent reactions of laser-ablated transition metals with small alkanes and halomethanes have introduced a variety of small transition-metal complexes showing interesting structures and photochemical reactions.9-15 Moreover, these transitionmetal reactions provide a very efficient means to form small high-oxidation-state complexes with C-M multiple bonds. While W, Re, and Os preferentially produce high-oxidationstate complexes with C-M triple bonds,9,11,12 the higher-oxidation-state complexes become less favored before and after them in the transition-metal rows in the periodic table.9-14 In this study, reactions of manganese with halomethanes have been investigated, and the products are identified in the matrix infrared spectra through isotopic substitution and with helpful predictions from DFT calculations for the plausible products. These Mn complexes bear the highest multiplicities among the group 3-12 metal complexes, and they provide simple models for manganese carbene complexes that can be explored theoretically. The insertion complexes resemble those of the group 2 metal analogues, the Grignard reagents.16

Experimental and Computational Methods Laser-ablated manganese atoms were reacted with CH3F (Matheson), 13CH3F, CH2F2, CH2FCl, CH2Cl2, CD2Cl2, 13 CH2Cl2 CHCl3, CDCl3, (MSD Isotopes), CFCl3, CF2Cl2 (Dupont), CCl4 (Fisher), 13CCl4 (90% enriched, MSD Isotopes), CD3F, CD2FCl, and CD2F2 (synthesized17a) in excess argon during condensation at 10 K using a closed-cycle refrigerator (Air Products Displex). These methods have been described in detail in previous publications.9,17b,c Reagent gas mixtures are typically 0.5% in (9) (a) Andrews, L.; Cho, H.-G. Organometallics 2006, 25, 4040, and references therein (review article, groups 4-6). (b) Lyon, J. T.; Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 2519 (Ti, Zr, Hf þ CHX3, CX4). (c) Lyon, J. T.; Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 6373 (Cr, Mo, W þ CHX3, CX4). (10) (a) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2007, 111, 2480. (b) Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 633 (group 3). (11) (a) Cho, H.-G.; Andrews, L. Organometallics 2007, 26, 4098. (b) Cho, H.-G.; Andrews, L. Inorg. Chem. 2008, 47, 1653 (Re). (12) (a) Cho, H.-G.; Lyon, J. T.; Andrews, L. Organometallics 2008, 27, 5241. (b) Cho, H.-G.; Andrews, L. Eur. J. Inorg. Chem. 2008, 2537. (c) Cho, H.-G.; Andrews, L. Organometallics 2008, 27, 1786 (group 8). (13) (a) Cho, H.-G.; Andrews, L. Organometallics 2009, 28, 5623 (Ni þ CX4). (b) Cho, H.-G.; Andrews, L.; Vlaisavljevich, B; Gagliardi, L. Organometallics 2009, 28, 6871 (Pd þ CX4). (c) Cho, H.-G.; Andrews, L. J. Am. Chem. Soc. 2008, 130, 15836. (d) Cho, H.-G.; Andrews, L. Organometallics 2009, 28, 1358. (e) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2008, 112, 12293 (group 10). (14) (a) Cho, H.-G.; Andrews, L. Organometallics 2010, 29, 2211 (Rh þ CX4). (b) Cho, H.-G.; Andrews, L. Dalton Trans. 2010, 39, 5478 (Ir þ CX4). (c) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2010, 114, ASAP (Co þ CX4). (15) (a) Andrews, L.; Cho, H.-G. J. Phys. Chem. A 2005, 109, 6796. (b) Cho, H.-G.; Lyon, J. T.; Andrews, L. J. Phys. Chem. A 2008, 112, 6902. (c) Lyon, J. T.; Cho, H.-G.; Andrews, L. Eur. J. Inorg. Chem. 2008, 1047 (actinides). (16) Bare, W. D.; Andrews, L. J. Am. Chem. Soc. 1998, 120, 7293. (17) (a) Isotopic modifications synthesized: Andrews, L.; Willner, H.; Prochaska, F. T. J. Fluorine Chem. 1979, 13, 273. (b) Andrews, L.; Citra, A. Chem. Rev. 2002, 102, 885, and references therein. (c) Andrews, L. Chem. Soc. Rev. 2004, 33, 123, and references therein.

Cho and Andrews Table 1. Frequencies of Product Absorptions Observed from Reactions of Mn with Fluoromethane Isotopomers in Excess Argona group

CH3F

CD3F

i

1157.6 635.0, 628.8 576.0, 574.4

918.4 628.3, 622.2 441.0, 436.6

13

CH3F

1149.0 633.4, 627.4 572.5, 570.6

description A1 CH3 deform A1 Mn-F stretch E CH3 rock

a All frequencies are in cm-1. The stronger matrix site split absorption is bold. Description gives major vibrational coordinate. A1 and E denote mode symmetry.

argon. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused on a rotating metal target (Mn, Johnson-Matthey) using 5-10 mJ/pulse. After initial reaction, infrared spectra were recorded at 0.5 cm-1 resolution using a Nicolet 550 spectrometer with a Hg-Cd-Te range B detector. Then samples were irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed using a combination of optical filters and annealed to allow further reagent diffusion. To provide support for the assignment of new experimental frequencies and to correlate with related works,9-15 density functional theory (DFT) calculations were performed using the Gaussian 03 program system,18 the B3LYP density functional,19 and the 6-311þþG(3df,3pd) basis sets for H, C, F, Cl, and Mn20 to provide vibrational frequencies for the reaction products. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. The BPW91 functional21 was also employed to complement the B3LYP results. The vibrational frequencies were calculated analytically, and zero-point energy is included in the calculation of binding and reaction energies. Previous investigations have shown that DFT-calculated harmonic frequencies are usually slightly higher than observed frequencies,9-15,22,23 and they provide useful predictions for infrared spectra of new molecules.

Results and Discussion The matrix infrared spectra from reaction products of laser-ablated manganese atoms with halomethanes have been investigated, and the observed frequencies (Tables 1-4) and their computed structures will be presented in turn. The DFTcomputed frequencies of the products are compared with the observed values (Tables S1-S16), and parameters from natural (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (19) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, Y.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (20) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062. (21) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum: New York, 1998. (22) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (23) Andersson, M. P.; Uvdal, P. L. J. Phys. Chem. A 2005, 109, 2937.

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Table 2. Frequencies of Product Absorptions Observed from Reactions of Mn with Dihalomethanes in Excess Argona group

CH2F2

CD2F2

CH2FCl

CD2FCl

i

921.1 642.5, 639.9 558.6

910.3, 906.8, 638.0, 636.2

546.3, 540.0 651.2, 650.0, 636.5 526.9

539.0 649.8, 642.0

746.1 740.7 626.9 589.2

m

736.4 584.1, 583.5 603.7

693.9 700.6 609.5

682.0 581.8

CH2Cl2 564.3 542.1 536.7 459.7, 457.5, 455.3 736.6 589.2 600.9

CD2Cl2 545.3, 541.3 510.8 577.0 507

13

CH2Cl2

551.8 539.0 525.1 459.5, 457.4, 455.0 covered 563.9

description A0 C-X str. A0 Mn-X str. A00 CH2 rock A0 C-M str. MnX2 as. str. CH2 wag C-Mn str. CH2 rock

All frequencies are in cm-1. Stronger absorptions in a set are bold. Description gives major vibrational coordinate: str. = stretch; as. str. = antisymmetric stretch. a

Table 3. Frequencies of Product Absorptions Observed from Reactions of Mn with Trichloromethane Isotopomers in Excess Argona group

CHCl3

m

586.7 478.5 436.6 501.2, 491.5

h a

CDCl3 459.9 428 477.3, 468.1

description C-H oop bend C-Mn str. MnCl2 as. str. higher order complex

All frequencies are in cm-1. Description gives major coordinate.

bond order analysis for the Mn-C bond of the insertion products are given (Table S17) as Supporting Information. A number of bands are common to laser-ablated metal experiments with each precursor, and these are due to precursor photolysis products that have been identified previously.9-15 For each precursor the observed bands are grouped by photolysis and annealing behavior, and the vibrational modes are assigned on the basis of isotopic shifts and correlation with DFT-calculated vibrational frequencies and intensities for a particular new product molecule. Mn þ CH3F. The product absorptions all marked “i” (for insertion product) from reactions of Mn with CH3F isotopomers are shown in Figure 1. They increase ∼5% and ∼20% on visible and UV irradiations and gradually decrease on annealing. This information shows that irradiation is required to initiate further reaction and that cold reagents do not react on annealing the matrix sample. The frequencies of the observed product absorptions are listed in Table 1 and compared with calculated frequencies for the anticipated CH3-MnF product in Table S1. No Mn-H stretching absorptions expected near 1600 cm-1 are observed in these Mn þ CH3F spectra nor in other Mn product spectra reported here, indicating that products with Mn-H bonds are not generated in the reactions with halomethanes.24 Deuteration and 13C substitution shift the i absorption at 1157.6 cm-1 to 918.4 and 1149.0 cm-1 (H/D and 12/13 ratios of 1.260 and 1.007), which are characteristic of a CH3 deformation mode of the insertion complex (CH3-MnF). The strong i absorption at 635.0 cm-1 with small isotopic shifts is assigned to the Mn-F stretching mode, and another absorption at 574.4 cm-1 is appropriate for the CH3 rocking mode. CH3-MnF has the C3v structure shown in Figure 2, which is similar to that of CH3-MgCl,25 but in contrast to the Cs structures for previously reported transition-metal insertion complexes.9-15 CH3-MnF, therefore, has four parallel (A1) and four perpendicular (E) bands. The observed bands are the strongest ones computed for CH3-MnF, and the observed frequencies are (24) Andrews, L.; Wang, X. J. Phys. Chem. A 2003, 107, 4081 Mn þ H2 . (25) (a) Kauffmann, T.; Bisling, M. Tetrahedron Lett. 1984, 25, 293. (b) Cahiez, G.; Alami, M. Tetrahedron Lett. 1986, 27, 569 Grignard reagent.

0.972, 0.988, and 0.999 of the B3LYP-computed frequencies as shown in Table S1, substantiating formation of this C-F bond activation insertion complex. The Grignard-type primary product is also the most stable among the plausible ones. CH3-MnF (S for sextet, 6A1) and CH2dMnHF (Q for quartet) are 58 and 8 kcal/mol lower energy than the ground-state reactants (Mn(6S) þ CH3F), while CH2F-MnH (singlet) is 6 kcal/mol higher. The unobserved higher energy CH2dMnHF (Q) complex shows no evidence of agostic distortion, as illustrated in Figure 2, in contrast to the analogous group 4, 5, and 6 methylidenes, but the H atom bonded to Mn is tilted back toward carbon, similar to the case of the Fe analogue.12 Clearly, H migration from C to Mn to form CH2dMnHF is not energetically favorable. Evidently the stability of a small methylidene complex carrying an Mn-H bond relative to the insertion complex decreases on going to the right side of the periodic table in the first-row transition metals. CH2dMHX is observed in the Sc, Ti, and V matrix spectra along with CH3-MX during co-deposition with methyl halides and on subsequent photolysis, whereas only the insertion products are identified in later first-row transitionmetal experiments.6,9-14,27 It is also notable that Re, another group 7 metal, exclusively produces methylidyne complexes via H migration in reactions with small alkanes and halomethanes.11 Mn þ CH2X2. Product absorptions from reactions of Mn with dihalomethane (CH2F2, CH2FCl, and CH2Cl2) isotopomers are given in Table 2. Both methylidene (m) and insertion (i) complex absorptions are observed unlike the fluoromethane (CH3F) case. The CH2F2 isotopomer spectra in Figure 3 show the i and m absorptions increase ∼35% and ∼25% on visible irradiation, but they remain unchanged on subsequent UV irradiation. They sharpen in the early stages of annealing but decrease on higher annealings. The product absorption at 699.5 cm-1 observed in the spectra with difluoromethane does not show intensity variation upon photolysis, unlike other Mn product absorptions, and agrees with the 699.4 cm-1 band identified as the antisymmetric stretching mode of the MnF2 molecule isolated in solid argon.28a Hence, Mn atoms clearly activate the C-F bond in fluoromethanes. The i absorption at 921.1 cm-1, with D counterpart at 906.8 cm-1, is assigned to the C-F stretching mode of CH2FMnF. The strongest i absorption is observed at 639.9 cm-1 along with its D counterpart at 636.2 cm-1, and it is assigned to the Mn-F stretching mode. Mode analysis shows that the Mn-F stretching mode coupled with the C-Mn stretching mode, (26) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899, and references therein. (27) Chang, S.-C.; Hauge, R. H.; Kafafi, Z. H.; Margrave, J. L.; Billups, W. E. J. Chem. Soc., Chem. Commun. 1987, 1682. (28) (a) Van Leirsburg, D. A.; DeKock, C. W. J. Phys. Chem. 1974, 78, 134. (b) Vogt, N. J. Mol. Struct. 2001, 570, 189 MnF2.

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Table 4. Frequencies of Product Absorptions Observed from Reactions of Mn with Tetrahalomethanes in Excess Argona group

CF4

i 947.0 674.9 covered m m0

720.7

CF3Cl

13

CF3Cl

1078.4 (CF3-MnCl) covered (CF3-MnCl)

1050.5 ( CF3-MnCl) covered (13CF3-MnCl)

415 (CF3-MnCl) 1299.6 (CF2dMnFCl) covered (CF2dMnFCl) 649.9 (CF2dMnFCl) 1226.4 (CF2-MnFCl) covered (CF2-MnFCl) 622.6 (CF2-MnFCl)

415 (13CF3-MnCl) 1267.3 (13CF2dMnFCl) covered (13CF2dMnFCl) 649.9 (13CF2dMnFCl) 1195.5 (13CF2-MnFCl) covered (13CF2-MnFCl) 622.5 (13CF2-MnFCl)

CCl4

13

CCl4

13

860.9 460.8, 458.3

834.3 460.8, 458.2

description CX3 str. CX3 str. CF3 deform Mn-F str. CX2 s. str. CX2 as. str. MnX2 as. str. CX2 s. str. CX2 as. str. MnX2 str.

a All frequencies are in cm-1. Stronger absorptions in a set are bold. Description gives major vibrational coordinate: str. = stretch; s. = symmetric, a. = antisymmetric. The m and m0 labels denote methylidene isomers.

Figure 1. Infrared spectra in the 1200-800 and 700-500 cm-1 regions for the reaction products of laser-ablated manganese atoms with methyl fluoride isotopomers (0.5% in argon) at 10 K. (a) Mn and CH3F co-deposited for 1 h, (b) after visible (λ > 420 nm) irradiation, and (c) after UV (240-380 nm) irradiation. (d) Mn and CD3F co-deposited for 1 h. (e, f) Spectra taken after visible and UV irradiations. (g) Mn and 13CH3F co-deposited for 1 h and (h) after UV irradiation. i stands for the product absorption. HCF is produced in CH3F reactions due to the ablation plume irradiation. P and c designate the precursor and common absorptions.

making it the C-Mn-F antisymmetric stretching mode. The C-Mn-F symmetric stretching mode expected near 470 cm-1 is, however, not observed due to low intensity. Another i absorption at 558.6 cm-1 is designated as the CH2 rocking mode. The observed absorptions are the strongest bands predicted for the C-F insertion complex as shown in Table S2, and the observed frequencies correlate with the B3LYP values of 923.4, 643.3, and 562.0 cm-1, substantiating the generation of CH2F-MnF. The strongest m absorption observed at 746.1 cm-1 with D counterpart at 736.4 cm-1 is assigned to the antisymmetric MnF2 stretching mode of CH2dMnF2. The second m absorption is observed at 740.7 cm-1, and deuteration shifts it to 583.5 cm-1 (H/D ratio of 1.269) for the CH2 wagging mode. The m absorption at 626.9 cm-1 has a D counterpart at 603.7 cm -1, and it is assigned to the C-Mn stretching mode, which is coupled with the symmetric MnF2 stretching mode. These three m absorptions support the formation of CH2dMnF2. Note that the B3LYP and BPW91 functional antisymmetric MnF2 stretching mode calculations bracket the observed value (Table S3), which is in satisfactory agreement for this high-multiplicity transition-metal species.

The identified CH2F-MnF(S) and CH2dMnF2(Q) are 43 and 48 kcal/mol more stable than the ground-state reactants (Mn(6S) þ CH2F2), and we find a sextet transition state CH2(F)-MnF 25 kcal/mol lower than reactants but 18 kcal/mol higher than the sextet insertion product and 6 kcal/mol higher than the excited CH2-MnF2(sextet), which ultimately relaxes to the CH2dMnF2(Q) ground state. In contrast, the excited CH2F-MnF(Q), CH2dMnF2(doublet), and CH2-MnF2(sextet) are only 8, 11, and 31 kcal/mol more stable, respectively, than reactants. The sextet ground state of CH2F-MnF and its almost linear C-Mn-F moiety shown in Figure 2 resemble the structure of CH3-MnF. Moreover, our computed structures show that the Mn insertion products all carry linear or near linear C-Mn-X moieties, and they all have the sextet ground states. In the Mn þ CH2FCl spectra, Figure S1, both the i and m absorptions remain unchanged on visible irradiation, but the i absorptions double on UV irradiation and the m absorptions increase ∼30%. The strongest i absorption is observed at 650.0 cm-1 along with its D counterpart at 546.3 cm-1 and is assigned to the Mn-F stretching mode of CH2Cl-MnF. Another i absorption at 546.3 cm-1 with D counterpart at 539.0 cm-1 is designated to the C-Cl stretching mode, and the absorption at 526.9 cm-1 is assigned to the CH2 rocking mode.

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Figure 2. B3LYP structures of the Mn insertion and methylidene products from reactions with mono- and dihalomethanes. The 6-311þþG(3df,3pd) basis sets are used for H, C, F, Cl, and Mn. Bond distances and angles are in A˚ and deg.

Figure 3. Infrared spectra in the 1000-500 cm-1 region for the reaction products of laser-ablated manganese atoms with CH2F2 and CD2F2 (0.5% in argon) at 10 K. (a) Mn and CH2F2 co-deposited for 1 h, (b) after visible (λ > 420 nm) irradiation, and (c) after UV (240-380 nm) irradiation. (d) Mn and CD2F2 co-deposited for 1 h, (e) after visible (λ > 420 nm) irradiation, and (f) after UV (240-380 nm) irradiation. i and m stand for the product absorption groups, while P and c designate the precursor and common absorptions.

The three observed absorptions are in fact the three strongest bands of CH2Cl-MnF (Table S5), substantiating formation of the insertion complex. The relatively low C-Cl

stretching frequency originates from the Cl-bridged structure of CH2Cl-MnF (Figure 2), leading to a weaker C-Cl bond. The Cl-bridged structure is in fact common for late transition-metal insertion complexes, particularly with C-Cl bonds.12-14 While CH2Cl-MnF is clearly observed in the Mn þ CH2FCl spectra, CH2F-MnCl is not identified. The strong C-F stretching absorption for CH2F-MnCl expected at ∼900 cm-1 is not observed, and the product frequencies do not agree with the predicted values for CH2F-MnCl. The complexes with an M-Cl bond are normally favored over the conformers with an M-F bond, due to the stronger C-F bond than the C-Cl bond.9-15 Evidently the preference is reversed in this Mn case. CH2Cl-MnF is favored over CH2F-MnCl in preference for the structure with bridging Cl (Figure 2), which is also consistent with the computations showing that CH2F-MnCl and CH2Cl-MnF are 52 and 56 kcal/mol more stable than the reactants (Mn (6S) þ CH2FCl). The strongest m absorption in the CH2FCl spectra at 693.9 cm-1 with D counterpart at 682.0 cm-1 is assigned to the Mn-F stretching mode of CH2dMnFCl. Another m absorption on the blue side at 700.6 cm-1 shifts to 581.8 cm-1 on deuteration, which is appropriate for the CH2 wagging mode. On the low-frequency side, the m absorption at 609.5 cm-1 is assigned to the C-Mn stretching mode. The observed m frequencies are in good agreement with the DFT values for planar CH2dMnFCl (55 kcal/mol more stable than the reactants), as shown in Table S6, which is also energetically comparable with CH2Cl-MnF.

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Figure 4. Infrared spectra in the 620-420 cm-1 region for the reaction products of the laser-ablated manganese atom with CH2Cl2 isotopomers (0.5% in argon) at 10 K. (a) Mn and CH2Cl2 co-deposited for 1 h, (b) after visible (λ > 420 nm) irradiation, and (c) after UV (240-380 nm) irradiation. (d) Mn and CD2Cl2 co-deposited for 1 h, (e) after visible (λ > 420 nm) irradiation, and (f) after UV (240-380 nm) irradiation. (g) Mn and 13CH2Cl2 co-deposited for 1 h, (h) after visible (λ > 420 nm) irradiation, and (i) after UV (240-380 nm) irradiation. i and m designate the product absorption groups. The absorptions assigned to MnCl2 are also shown.

Figure 4 shows product absorptions from reactions of Mn with CH2Cl2 isotopomers, where both i and m absorptions are observed. The i absorptions increase 15% and 30% on visible and UV irradiations, respectively, and decrease on annealing. The strongest i absorption is observed at 564.3 cm-1, and deuteration and 13C substitution shift it to 545.3 and 551.8 cm-1 (H/D and 12/13 ratios of 1.034 and 1.023). It is assigned to the C-Cl stretching mode of CH2Cl-MnCl. The insertion complex also has a bridged structure, resulting in the relatively low C-Cl stretching frequency. The i absorption at 542.1 cm-1 has its 13C counterpart at 539.0 cm-1 for the CH2 rocking mode. On the red side, another i absorption is observed at 536.7 cm-1, with D and 13C counterparts at 510.8 and 525.1 cm-1, and is assigned to the C-Mn stretching mode. The observed i absorptions are the strongest bands for CH2Cl-MnCl (Table S7). The absorptions at 476.4, 473.4, and 471.1 cm-1 observed in the CH2Cl2 isotopomer spectra with an intensity ratio of 9:6:1 (the statistical ratio for 2  35Cl, 35Cl þ 37Cl, and 2  37 Cl in natural abundance for the vibration of two equivalent chlorine atoms) are appropriate for the MnCl2 product of the reaction of Mn with halomethanes containing at least two Cl atoms: this chlorine isotopic triplet for MnCl2 agrees (within 0.5 cm-1) with previous argon matrix spectra.29a The formation of MnCl2 confirms activation of the C-Cl bond by Mn in these experiments. (29) (a) Thompson, K. R.; Carlson, K. D. J. Chem. Phys. 1968, 49, 4379. (b) Hargittai, M.; Subbotina, N. Y.; Kolonits, M.; Gershikov, A. G. J. Chem. Phys. 1991, 94, 7278 MnCl2.

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On the other hand, the m absorptions decrease slightly on visible and UV irradiations, respectively. The strongest m absorption is observed at 736.6 cm-1 in an area congested with precursor absorptions (not shown), with D counterpart at 577.0 cm-1 (H/D ratio of 1.277). It is designated as the CH2 wagging mode of CH2dMnCl2. The m absorption at 600.9 cm-1 with its D counterpart at 507 cm-1 (H/D ratio of 1.185) is assigned to the CH2 rocking mode. The m absorption at 589.2 cm-1 is assigned to the C-Mn stretching mode. A triplet of m absorptions at 459.7, 457.5, and 455.3 cm-1 with an intensity ratio of 9:6:1 again for the vibration of two equivalent chlorine atoms falls just below that for isolated MnCl2 and shows negligible 13C shifts. These bands are assigned to the MnCl2 antisymmetric stretching mode of CH2dMnCl2. The observed frequencies are in good correlation with the B3LYP values (Table S8), which substantiates the formation of CH2dMnCl2. Mn þ CHCl3. Shown in Figure S2 are the product absorptions in the infrared spectra from reactions of Mn with CHCl3 and CDCl3. The relatively weak product absorptions (all marked with “m”) halve on visible irradiation and recover on UV and full arc irradiations. No insertion product absorptions are identified, but their computed frequencies are listed in Table S9. Another group of product absorptions marked “h” increase gradually during photolysis and significantly in the early stage of annealing. They probably originate from a higherorder or aggregate product. The m absorptions are tentatively assigned to CHCldMnCl2(Q) (Table S10). The m absorptions at 586.7 and 436.6 cm-1 along with their D counterparts at 459.9 and 428 cm-1 are assigned to the C-H out-of-plane bending and MnCl2 antisymmetric stretching modes of CHCldMnCl2. CHCldMnCl2(Q) is also slightly more stable than CHCl2MnCl(S) (68 and 66 kcal/mol more stable than the reactants), but the insertion product is not observed here. Mn þ CX4. Figure S3 compares infrared spectra from reactions of Mn atoms with CF4, CF3Cl, and 13CF3Cl. In the CF4 spectra, the product absorptions show essentially no change in the process of photolysis. The i absorption observed at 947.0 cm-1 is assigned to the antisymmetric CF3 stretching mode of CF3-MnF. This insertion complex bears a C3v structure, as shown in Figure 5, and the strongest infrared absorption is the above mode computed at 998.2 cm-1 (Table S11). The weaker symmetric counterpart computed at 1145.7 cm-1 is probably masked by the CF2 photolysis product band at 1102 cm-1.30 In the low-frequency region a weaker i absorption is observed at 674.9 cm-1 for the CF3 deformation mode. The Mn-F stretching absorption expected at ∼620 cm-1 is probably covered by the CF4 precursor absorption at this position. These two observed absorptions support the formation of CF3-MnF. The weak m absorption observed in the MnF2 region at 720.7 cm-1 is tentatively assigned to the MnF2 antisymmetric stretching mode of CF2dMnF2 on the basis of good agreement with the DFT value (e.g., B3LYP frequency 748.9 cm-1 in Table S12) and its appearance just above MnF2 at 699.5 cm-1, while the CF2 stretching absorptions are not observed, likely due to masking by strong C-F stretching absorptions common to the CF4 experiments with laserablated metals. Importantly CF3-MnF and CF2dMnF2 in their sextet and quartet ground states are 43 and 38 kcal/mol more stable than the reactants. (30) Jacox, M. E. J. Phys. Chem. Ref. Data 1994, Monograph 3; 1998, 27 (2), 115, Supplement B, 2003, 32, 1.

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Figure 5. B3LYP structures of the Mn insertion and methylidene products from reactions with tri- and tetrahalomethanes. The 6-311þþG(3df,3pd) basis sets are used for C, F, Cl, and Mn. Bond distances and angles are in A˚ and deg. The Cl-C-Mn-Cl dihedral angles for CHCldMnCl2 and CCl2dMnCl2 are also shown.

In the CF3Cl and 13CF3Cl spectra, new absorptions are observed, marked “i, m, and m0 (for m structural isomer)”, respectively, and they are assigned to CF3-MnCl(sextet), CF2dMnFCl(quartet), and CF2-MnFCl(sextet). The i absorptions increase ∼0%, ∼10%, and ∼20% on visible, UV, and full arc photolysis and gradually decrease on annealing. The m absorptions increase together ∼0%, ∼10%, and ∼10% on visible, UV, and full arc photolysis and gradually decrease on annealing. The m0 absorptions increase ∼10%, ∼50%, and ∼40% on visible, UV, and full arc photolysis, respectively, and sharpen in the early stage of annealing. The i absorptions observed at 1078.4 (shoulder) cm-1 and the 13C counterparts at 1050.5 (shoulder) and 975.0 cm-1 are assigned to the CF3 stretching mode of CF3-MnCl (Table S14). Another i absorption is observed at 415 cm-1 (not shown), with no 13C shift, and it is assigned to the Mn-Cl stretching mode. These are the strongest observable bands for the insertion complex with a C3v structure shown in Figure 5 (Table S13). The m absorption at 1299.6 cm-1 carries a 13C counterpart at 1267.3 cm-1, and its relatively high frequency suggests a higher-oxidation-state complex, leading to an assignment to the CF2 symmetric stretching mode of CF2dMnFCl in the quartet state. The weak m absorption at 649.9 cm-1 shows a negligible 13 C shift, and therefore, it is assigned to the Mn-F stretching mode. The m0 absorption at 1226.3 cm-1 has its 13C counterpart at 1195.5 cm-1 and is tentatively assigned to the CF2 symmetric stretching mode of the distorted CF2-MnFCl molecule in the sextet state as shown in Table S15. The CF2 antisymmetric

stretching absorption expected near 1165 cm-1 is most probably covered by precursor absorption in this congested area. A strong m0 absorption is observed at 622.5 cm-1 in the relatively clean region and shows negligible 13C shift. It is assigned to the Mn-F stretching mode. These conformers (CF3-MnCl, CF2dMnFCl, and CF2-MnFCl) are energetically comparable (60, 51, and 57 kcal/mol more stable than the reactants, respectively), and trapping of both of the last two is reasonable owing to their different structures. Figure 6 shows the product spectra from reactions of Mn with CCl4 and 13CCl4, where only m absorptions are obmediate species common to experiments with CCl4.12-14,30 The m absorptions increase approximately 30% and 70% on visible and UV photolysis and decrease on annealing. The broad m absorption at 860.9 cm-1 shifts to 834.3 cm-1 on 13C substitution (12/13 ratio of 1.032). In the low-frequency region, another m absorption is observed at 460.9 cm-1 (with a shoulder at 458.3 cm-1), and it shows a negligible 13C shift. This band is just below that for MnCl2 and CH2dMnCl2 assigned above, and the shoulder is appropriate for the mixed chlorine-35,37 component in the triplet for two equivalent chlorine atoms. They are assigned to the CCl2 and MnCl2 antisymmetric stretching modes of CCl2dMnCl2 on the basis of the frequencies and appropriate 13C shifts for these modes. These are the strongest bands for the Mn tetrachloro methylidene, and the observed frequencies are in reasonable agreement with the DFT values shown in Table S16. The observed CCl2 stretching frequencies higher than the DFT

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Figure 6. Infrared spectra in the 1000-800 and 600-400 cm-1 regions for the reaction products of the laser-ablated manganese atom with CCl4 and 13CCl4 (0.5% in argon) at 10 K. (a) Mn and CCl4 co-deposited for 1 h, (b) after visible (λ > 420 nm) irradiation, and (c) after UV (240-380 nm) irradiation. (d) Mn and 13CCl4 co-deposited for 1 h, (e) after visible (λ > 420 nm) irradiation, and (f) after UV (240-380 nm) irradiation. m designates the methylidene product absorption. The CCl3 and Cl2-CCl-Cl absorptions are produced from the precursor by ablation plume radiation.

values are not unusual for the tetrahalo transition-metal carbenes and carbynes probably due to underestimation of the stretching frequencies by the DFT methods.12-14 The CCl2dMnCl2 carbene and the unidentified CCl3-MnCl in the quartet and sextet ground states are energetically comparable (78 and 74 kcal/mol more stable than the reactants). However, CCl3-MnCl would show a strong CCl3 antisymmetric stretching band near 640-620 cm-1, based on DFT calculations, which is not observed in this study. Structure and Bonding in Small Mn Complexes. The structures of the Mn insertion and carbene complexes investigated here are illustrated in Figures 2 and 5. These Mn insertion and carbene complexes are computed to have sextet and quartet ground states in line with the quintet and triplet grounds states for the previously studied Cr and Fe insertion and carbene products.9,12 The Re carbyne complexes, which are exclusively produced in reactions with small alkanes and halomethanes, have doublet ground states.11 These Mn insertion and methylidene complexes, therefore, have the highest multiplicities among the group 3-12 metal complexes produced from reactions with small alkanes and halomethanes. It is also interesting that the Mn insertion complexes have a linear or near linear C-M-X moiety, parallel to that of the Grignard reagent,16,25 and high Mn s character in the C-Mn single bond based on NBO calculations (Table S17). Among the previously studied small transition-metal insertion complexes, only the Fe products have the linear C-M-X moiety.9-14 The CH3-MnF, CF3-MnF, and CF3-MnCl complexes have C3v structures as illustrated in Figures 2 and 5, similar to the Fe counterparts.12a However, the Mn insertion complexes with a C-Cl bond all have bridged structures. No Mn product with bridging F, similar to CH2F-NiCl, is observed, reconfirming that the larger more

polarizable Cl is more effective than F to form a bridged structure between C and Mn. The sextet ground state along with the C3v structure of CH3-MnF indicate promotion of a 4s electron, and NBO26 calculations show a contribution of s (65.7%) and d (33.7%) from Mn in the Mn-C bond. In comparison, the Mg-C bond of CH3-MgCl has mostly s character (97.9%) from Mg. In order to supplement the DFT-based NBO calculations, a complete active-space self-consistent-field (CASSCF) (9,9) calculation was done for CH3-MnF. The nine orbitals are 1 from C and 1 from Mn forming σ and σ* C-M bonds, 5 3dorbitals from Mn, 1 p-orbital from F (2pz), and 1 p-orbital from Mn (4pz) mixed with other orbitals forming a C, Mn, F molecular orbital. The 9 electrons (1 from C, 7 from Mn, 1 from F) occupy the 9 CAS orbitals illustrated in Figure 7, and the given occupation numbers are consistent with the NBO results. The C-Mn bond order [bonding minus antibonding electron occupancies divided by 2] is 0.97. In addition, these calculations show that one electron essentially moves from Mn toward F, occupying the 2pz-orbital of F (the two other 2p-orbitals of F, not shown, are also doubly filled). This result demonstrates that CH3-MnF is basically a single-configuration problem. The Mn methylidene complexes identified here are mostly planar like the previously studied early transition-metal methylidene complexes through Cr.9,10 In contrast the late transition-metal methylidene complexes including Fe have instead staggered allene-type structures.12-14 The unidentified Re methylidene complexes would also have staggered structures, suggesting that these small group 7 complexes show borderline characteristics between the early and late transition-metal compounds. CHCldMnCl2, on the other hand, has a near planar structure (Φ(ClCMnCl) = 12.0°), probably due to repulsion between large Cl atoms bonded to C

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Figure 7. CASSCF orbitals using a (9,9) active space for CH3-MnF calculated at the B3LYP minimum and plotted with isodensity 0.02 e/A˚3. The occupation numbers are given in parentheses: the C-Mn bond order is 0.97. The z axis is the molecular axis, the molecular plane is the yz plane, and the x axis is perpendicular to the molecular plane.

and to Mn, leading to distortion along the C-Mn bond. More distortion along the C-Mn bond is observed in CCl2dMnCl2, with a C2 structure and Cl-C-Mn-Cl dihedral angle of 53.4°. The computed C-Mn bond lengths of the carbene complexes in the quartet states (1.855-1.872 A˚) are considerably shorter than those of the insertion complexes in the sextet states (2.057-2.120 A˚) and bond lengths measured for ligated manganese carbene complexes (2.068 and 2.227 A˚).8a,c NBO calculations26 for the methylidenes produced ionic fluoride structures and natural bond orders for the C-Mn bonds in CH2d MnF2, CH2dMnCl2, and CCl2dMnCl2 of 1.42, 1.37, and 1.30, respectively. In order to explore bonding in these carbene complexes more extensively, CASSCF calculations18 were done for the C-Mn bonds in CH2dMnF2, CH2dMnCl2, and CCl2dMnCl2 like those recently reported for CCl2dCoCl214c but adding two more d-orbitals. This active space employed 7 electrons (2 from C and 5 from Mn, the 2 other electrons from Mn ultimately go into F orbitals) in 8 orbitals (2 from C and 6 from Mn) (CAS(7,8)). The resulting molecular orbitals and their occupancies are given below: On the basis of the CAS orbital shapes and energies, the six MOs from Mn are mostly 4sand five 3d-orbitals. The 2s- and 2p-orbitals from C and the 4sand two 3d-orbitals from Mn constitute the active space along with the remaining three 3d-orbitals of Mn, each of which contains one electron in the quartet state. These form the given σ, π, σ*, π*, 3dxy, 3dxz, 3dz2 and Mn,C,H,F molecular orbitals (very small electron density shared over these atoms in the latter). The CAS(7,8) molecular orbitals are illustrated in Figure S5 for CH2dMnF2. CH2dMnF2 σ (1.854), π (1.527), 3dxy (0.996), 3dxz (1.001) σ* (0.145), π* (0.472), Mn,C,H,F (0.005), 3dz2 (1.000) CH2dMnCl2 σ (1.803), π (1.515), 3dxy (0.996), 3dxz (1.000) σ* (0.198), π* (0.485), Mn,C,H,Cl (0.005), 3dz2 (1.000) CCl2dMnCl2 σ (1.822), π (1.547), 3dxy (0.995), 3dxz (1.000) σ* (0.178), π* (0.452), Mn,C,H,Cl (0.005), 3dz2 (1.000) The net C-Mn bond orders are 1.39, 1.32, and 1.37, which reveal substantial π-character for these C-Mn bonds.

Figure 8. CASSCF orbitals using an (11,12) active space involved in the carbon-manganese bond for CH2dMnF2 calculated at the B3LYP minimum and plotted with isodensity 0.02 e/A˚3. The occupation numbers are given in parentheses: the C-Mn bond order is 0.86 for sigma and 0.55 for pi. The z-axis is perpendicular to the molecular plane, the C-Mn bond is on the y-axis, and xy is the molecular plane.

In order to be more complete and also to investigate the Mn-F bonding, the CAS for CH2dMnF2 was expanded to (9,10) by adding two more electrons and one unoccupied low-energy MO as well as one MO from a fluorine p-orbital. The energy decreased 2.7 kcal/mol, but the C-Mn bond order was virtually the same, 1.39. Next, the CAS was expanded to (11,12) by adding two more electrons and one more unoccupied low-energy MO as well as one more MO from fluorine p-orbitals. The choice of which two p-orbitals to include on the F atoms affects the orbital energy ordering but not the C-Mn bond order. For one choice of the two F p-orbitals, the energy decreased another 3.2 kcal/mol, and the C-Mn bond order is 1.40: these orbitals include two Mn-F2 orbitals and are shown in Figure S6. For another choice, the energy decreased a total of 7.3 kcal/mol. These CAS (11,12) molecular orbitals are shown for CH2dMnF2 in Figure 8, and the net C-Mn bond order is 1.41, essentially the same as for the smaller (7,8) active space. These results all demonstrate that inclusion of extra fluorine p-orbitals and unoccupied orbitals causes only negligible changes in the occupation numbers of the molecular orbitals involved in the C-Mn bond. There are six MOs from the F 2p-orbitals, and HF and B3LYP calculations show that they are all doubly occupied, indicating even in the single configuration calculations that two electrons indeed move from Mn toward the

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fluorine p-orbitals, resulting in ionic Mn-F bonding. However, the CAS optimization changes the order of the molecular orbital energies. While the original 8 MOs for CAS(7,8) remain essentially unchanged, the σ (C-H2) orbital gets into the active space, replacing an F 2p-orbital. In addition, the σ (Mn-F2) molecular orbital shows that the Mn-F bonding is not completely ionic (Figure 8). The CH2F-MnF bond order from NBO analysis (Table S17) is 0.93, and the bond is considerably longer (2.081 compared to 1.856 A˚). The higher-oxidation-state carbene complex also has more ionic character, as the Mulliken charges on C and Mn (Table S18, C 0.20, H -0.02, F (on C) -0.41, Mn 0.82, and F (on Mn) -0.58) indicate. So the multiple bond character in the methylidene complexes arises in part from ionic bonding as well as the π-character. (We have retained the double-bond notation for the Mn methylidenes where the second bond is ionic as well as pi, based on our calculations.) For comparison, previous CASSCF calculations determine a 1.67 effective bond order for the CCl2dCoCl2 complex in the doublet state,14c where there is more π-contribution to the bonding than in the Mn analogue CCl2dMnCl2. Finally, it is interesting to note that CF2-MnFCl in the sextet state with a bridged structure has the longest the C-Mn bond (2.265 A˚) among the investigated Mn complexes (Figure 5), and therefore, it is a single bond and this Mn carbene with a bridged structure is in fact an extreme case of Mn insertion complexes. Reactions. The new Mn-bearing reaction products are formed on reactions of laser-ablated Mn atoms on sample depositions and on subsequent photolysis, which suggest that these reactions require excited Mn atoms, a fact consistent with the failure to observe product growth on annealing of cold reagents. This is also in line with the earlier observation that thermal Mn atoms failed to react with pure solid methane nor water in excess argon, but CH3-MnH or HMnOH was formed on UV (300-340 nm) photolysis.6 Previous laser ablation investigations reveal the presence of Mn* (8P, 4s3d54p) metastable states 53 kcal/mol above the ground state (6S, 4s23d5), which are probably responsible for our reactions during sample deposition.31 Clearly, photolysis of the cold sample leads to reaction most likely through photoexcitation of Mn. Parallel to the previously studied transition-metal reactions with halomethanes, the Mn reaction is also believed to undergo C-X bond insertion to produce the insertion complex followed by X migration from C to M to generate the higher-oxidation-state complexes during sample co-deposition and on subsequent photolysis.9 The methylidyne complexes (CXtMnX3) are energetically much higher (e.g., FCtMnF3 in the doublet ground state is 8 kcal/mol higher than the total ground-state reactant energy), and as a result, no Mn carbynes are observed from the Mn reactions investigated. This is in contrast to the exclusive formation of Re methylidyne complexes.11 These results reveal that the tendency of the increasing preference for the higher-oxidation-state complexes on descending in the family group is most dramatic among group 7 metals along

with groups 6 and 8 metals.9,11,12 Reaction 1 summarizes the Mn reactions described here. The evidence suggests that excited Mn* forms an energized insertion complex, CX3MnX*, which can relax in the matrix to the ground sextet state, as observed in many cases, or rearrange through a halogenbridged transition state (Figure S4) to an excited sextet state CX2-MnX2. This latter intermediate is relaxed by the matrix to the ground quartet state methylidene final product.

(31) (a) Levy, M. R. J. Phys. Chem. 1989, 93, 5195. (b) Chertihin, G. V.; Andrews, L. J. Phys. Chem. A 1996, 101, 8547.

Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

The observed primary products are essentially in line with the product stabilities as described above. The methylidene product becomes relatively more stable than the insertion complex with increasing number of halogen atoms, particularly Cl, and as a result, only the insertion product is observed in the CH3F reactions and only the methylidene complex in the CCl4 system.

Conclusions Reactions of laser-ablated Mn atoms with halomethanes have been carried out, and the products are identified from the matrix IR spectra on the basis of frequencies, isotopic shifts, and correlation with the DFT-computed frequencies. In contrast to the Re case,11 no Mn carbynes are identified. Instead, only the insertion complex is observed in the CH3F spectra, whereas both the insertion and carbene products are identified in most other reactions, except for the CHCl3 and CCl4 cases, where only the carbenes are observed. The observed products have the lowest computed energies. The Mn insertion complexes have either a linear or near linear C-Mn-X moiety, similar to the Grignard reagents and the Fe analogues, and NBO analysis shows that Mn has a high s character (56-66%) in the single C-Mn bonds. The small Mn methylidene complexes mostly have planar structures, like the early transition-metal methylidenes,9,10 which is in contrast with the late transition-metal (including Fe and Re) methylidenes with staggered allene-type structures.12-14 The computed C-Mn bond lengths for the methylidenes are shorter than for the insertion complexes, which is explained by a one-half π-bond order and increased ionic bonding in the higher-oxidation-state species to augment the σ-bond based on CASSCF calculations. The linear C-Mn-X moiety of the Mn insertion complexes and the high s character in the Mn-X bond resemble those for group 2 metal analogues, such as the Grignard reagents. The present results reveal that these small Mn insertion and methylidene complexes carry the borderline characteristics between the early and late transition metals and also bear the highest multiplicities among the group 3-12 products. The tendency of the increasing preference for higher-oxidation-state complexes is also most dramatic among the group 7 metals (Mn and Re).

Acknowledgment. We gratefully acknowledge financial support from National Science Foundation (U.S.) Grant CHE 03-52487 to L.A. and support from the Korea Research Foundation (KRF) grant funded by the Korean government (MEST) (No 2010-0016527).