Infrared Spectra of CH 2 ═M(H)NC, CH 3 −MNC, and η 2 -M(NC)−CH 3 Produced by Reactions of Laser-Ablated Group 5 Metal Atoms with Acetonitrile

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<ul><li><p>Infrared Spectra of CH2dM(H)NC, CH3-MNC, and 2-M(NC)-CH3 Produced byReactions of Laser-Ablated Group 5 Metal Atoms with Acetonitrile</p><p>Han-Gook Cho and Lester Andrews*Department of Chemistry, UniVersity of Incheon, 12-1 Songdo-dong, Yonsu-ku, Incheon, 406-840, South Korea,and Department of Chemistry, UniVersity of Virginia, P.O. Box 400319, CharlottesVille, Virginia 22904-4319</p><p>ReceiVed: February 9, 2010; ReVised Manuscript ReceiVed: March 24, 2010</p><p>Methylidene isocyanides, methyl isocyanides, and 2-nitrile--complexes are observed in the matrix IR spectrafrom reactions of Group 5 metals with acetonitrile isotopomers. The primary isocyanide products with notrace of cyanide complexes are consistent with the reaction path proposed in the analogous Zr study. Themajor products (CH2dTa(H)NC, CH3-NbNC, 2-Nb(NC)-CH3, and 2-V(NC)-CH3) after codepositionand reaction of metal with CH3CN clearly show the increasing preference for the higher oxidation-state complexon going down the group column, and the subsequent photochemistry provides further information for molecularrearrangements. The Group 5 metal methylidene isocyanides exhibit more agostic distortion than the Zrcounterparts and are comparable to the previously studied Group 5 metal methylidene hydrides and halides.The computed structures and observed frequencies indicate that the effects of metal conjugation (CdTa-NdC:)are minor.</p><p>Introduction</p><p>Coordination of metal atoms to electron-rich organic speciesplays a pivotal role for further rearrangements to more sophis-ticated end products.1-3 Metal interactions with lone electronpairs and -electron systems provide driving forces for theformation of new metal bonds, leading to catalytic activitiesand further reactions such as C-H and C-C bond insertions.2-6Metal-participating molecular rearrangements often consist ofmultistep configurational changes involving distinct stationarypoints and transition states.6 Therefore, investigation of theprocess for electrophilic coordination of metal atoms and theirsubsequent molecular rearrangements is essential to understand-ing the details of the reaction path and eventually to synthesizemore precious chemical reagents.</p><p>Recent studies have shown that reactions of laser-ablated metalatoms with small hydrocarbons and haloalkanes are efficient routesto produce characteristic complexes including carbenes, carbynes,-complexes, and cyclic products.1-3,7-13 Although they are cousinsof the much larger complexes, their small sizes also allowopportunities to closely examine molecular processes involved inmetal coordination, bond insertion, H(X) migration, and photo-chemical reactions. Electronic structure calculations also offerhelpful information for spectral assignments and understanding thereaction mechanism.6,11,14 Studies show that Groups 3-10 metals,lanthanides, and actinides undergo C-H(X) bond insertion inreaction with small organic compounds and also produce highoxidation-state complexes15 either during reactions or upon pho-tolysis afterward.7-13</p><p>A more recent study shows that reactions of Zr with CH3CN,a well-known electron donor, yields methylidene isocyanide(CH2dZrHNC), methylisocyanide (CH3-ZrNC), and 2-nitrile--complex, with no trace of the corresponding cyanide products,and their energies are comparable.16 The primary productssuggest a reaction path that includes electrophilic coordinationof metal atom to the N-end of CH3CN, formation of the more</p><p>stable nitrile--complex, C-C bond insertion, and H migration.The observed products are the most stable computed speciesalong the reaction path, and the transition states between theproducts are examined. The extent of agostic distortion14 andthe observed frequencies indicate that the effects of the metalcontaining conjugation are minor.</p><p>In this investigation, reactions of Group 5 metal atoms withacetonitrile isotopomers are carried out in an effort to substanti-ate the previous Zr results. The primary products are identifiedthrough isotopic substitution and with helpful information fromDFT computations. The increasing preference for the higheroxidation-state product on going down the family column isclear, and the results are in line with the previously proposedreaction path for the Zr system.</p><p>Experimental and Computational Methods</p><p>Laser ablated Ta, Nb, and V atoms (Johnson-Matthey) werereacted with acetonitrile isotopomers (CH3CN, CD3CN, and13CH313CN) in excess argon during condensation at 10 K usinga closed-cycle refrigerator (Air Products Displex). Thesemethods have been described in detail in previous publications.17</p><p>Reagent gas mixtures are typically 0.5% in argon. The Nd:YAGlaser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulsewidth) was focused onto a rotating metal target using 5-10mJ/pulse. After initial reaction, infrared spectra were recordedat 0.5 cm-1 resolution using a Nicolet 550 spectrometer with aHg-Cd-Te range B detector. Then samples were irradiatedfor 20 min periods by a mercury arc street lamp (175 W) withthe globe removed using a combination of optical filters orannealed to allow further reagent diffusion.</p><p>To provide support for the assignment of new experimentalfrequencies and to correlate with related works,7-13 densityfunctional theory (DFT) calculations were performed using theGaussian 03 program system,18 the B3LYP density functional,19</p><p>the 6-311++G(3df,3pd) basis sets for H, C, N, and V20 andusing the SDD pseudopotential and basis sets21 for Nb and Tato provide vibrational frequencies for the reaction products.</p><p>* Author to whom correspondence should be addressed. E-mail:</p><p>J. Phys. Chem. A 2010, 114, 59976006 5997</p><p>10.1021/jp1012686 2010 American Chemical SocietyPublished on Web 04/28/2010</p></li><li><p>Geometries were fully relaxed during optimization, and theoptimized geometry and transition-state structure were confirmedby vibrational analysis. The BPW9122 functional was alsoemployed to support the B3LYP results. The vibrationalfrequencies were calculated analytically, and zero-point energyis included in the calculation of binding and reaction energies.Previous investigations have shown that DFT calculated har-monic frequencies are usually slightly higher than observedfrequencies,7-13,23 and they provide useful predictions for theinfrared spectra of new molecules.</p><p>Results and Discussion</p><p>Reactions of Group 5 metal atoms with acetonitrile wereinvestigated and infrared spectra (Figures 1-6), and densityfunctional frequency calculations of the products and theirrelative energies (Figures 7-9) and structures (Figures 10-12)will be presented in turn.</p><p>Ta + Acetonitrile. Figures 1-3 show the product spectrafrom reactions of laser-ablated Ta atoms with acetonitrileisotopomers and their variation with subsequent photolysis and</p><p>annealing. Isomerization of acetonitrile due to the laser-plumeirradiation during ablation produces CH2CNH, CH2NCH, andCH3NC absorptions in the matrix IR spectra.24 The productabsorptions are all marked with m (m for methylidene), whichshow photoreversible intensity variations upon visible ( &gt; 420nm) and UV (240 &lt; &lt; 380 nm) irradiations. They are observedmostly in pairs 1.1-14.0 cm-1 apart as listed in Table 1, andthe intensity variations of the components for a pair are oppositeeach other. The ones shown with boldface letters in Table 1increase and decrease on UV and visible irradiations, and theother ones with plain letters show intensity changes in theopposite directions. This suggests that at least two competingsites for the Ta product exist in the matrix, and photolysisswitches from one to the other depending on the photon energy,leading to the considerable intensity alterations as shown inFigures 1-3. Similar photoreversible intensity variations dueto different matrix sites are also observed from Group 4 metalreactions with small alkanes and halomethanes.7</p><p>Excellent agreement between the observed and DFT com-puted frequencies shown in Table 1 substantiates generation ofthe small Ta methylidene isocyanide, CH2dTa(H)NC. Thecorresponding cyanide complex (CH2dTa(H)CN) would havesimilar frequencies except for the C-N stretching band expectedat about 2150 cm-1, which is not observed in this study as shownin Figures 1-3. The strongest m absorption at 2027.5 cm-1(with a weak site absorption at 2024.2 cm-1) is assigned to theN-C stretching mode on the basis of the frequency and thenegligible D and substantial 13C (39.4 cm-1) shifts. On the otherhand, the m absorption observed at 1782.5 cm-1 (with a siteabsorption at 1788.7 cm-1) in the Ta-H stretching region showsa large D shift of 505.5 cm-1 (H/D ratio of 1.396) and anegligible 13C shift, leading to an assignment to the Ta-Hstretching mode. The observed Ta-H stretching frequency isalso compared with 1758.9 and 1732.9 cm-1 for TaH225 andthose for the previously studied Ta methylidenes (1753.8 and1731.9 cm-1 for CH2dTaH2, 1765.0 and 1759.3 cm-1 forCH2dTaHF, 1762.9 and 1759.6 cm-1 for CH2dTaHCl, and1760.3 cm-1 for CH2dTaHBr).7</p><p>A weak m absorption at 1012.9 cm-1 (with a site absorptionat 1005.3 cm-1) in the CD3CN spectra (Figure 2) has its 13Ccounterpart at 1297.3 cm-1, and it is assigned to the CD2scissoring mode while its H counterpart is probably coveredby the common CH4 absorption center at 1305 cm-1. Anotherm absorption is observed at 819.6 cm-1 (with a site absorptionat 823.6 cm-1), has its D counterpart at 744.6 cm-1, and has13C counterparts at 798.2 cm-1 (with a site absorption at 800.7</p><p>Figure 1. Infrared spectra in the 2200-1600 and 900-600 cm-1regions for the reaction products of the laser-ablated tantalum atomwith CH3CN in excess argon at 10 K. (a) Ta and CH3CN (0.50% inargon) codeposited for 1 h, (b) as (a) after visible ( &gt; 420 nm)irradiation, (c) as (b) after UV (240-380 nm) irradiation, (d) as (c)after visible irradiation, (e) as (d) after UV irradiation, and (f) as (e)after annealing to 28 K. m designates the product absorption, and Pand c stand for the precursor and common in the CH3CN matrix spectra.CH2CNH, CH2NCH, and CH3NC absorptions are also indicated.</p><p>Figure 2. Infrared spectra in the 2150-1850 and 1300-450 cm-1regions for the reaction products of the laser-ablated tantalum atomwith CD3CN in excess argon at 10 K. (a) Ta and CD3CN (0.50% inargon) codeposited for 1 h and (b-f) as (a) after visible, UV, visible,and UV irradiations and annealing to 28 K following the same sequencein Figure 1. m stands for the product absorption, and P and c designatethe precursor and common absorptions in the CH3CN matrix spectra.CD2CND and CD2NCD absorptions are also indicated.</p><p>Figure 3. Infrared spectra in the 2200-1700, 1350-1250, and850-450 cm-1 regions for the reaction products of the laser-ablatedtantalum atom with 13CH313CN in excess argon at 10 K. (a) Ta and13CH313CN (0.50% in argon) codeposited for 1 h and (b-f) as (a) aftervisible, UV, visible, and UV irradiations and annealing to 28 K followingthe same sequence in Figure 1. m stands for product absorption, and Pand c designate the product and common absorptions. 13CH2CNH,13CH2N13CH, and H13C13CNH2 absorptions are also indicated.</p><p>5998 J. Phys. Chem. A, Vol. 114, No. 19, 2010 Cho and Andrews</p></li><li><p>cm-1). Due to the modest H/D and relatively large 12/13 ratiosof 1.101 and 1.027, it is designated to be the C-Ta stretchingmode, and its relatively high carbon-metal stretching frequencyindicates that it is a carbon-tantalum double bond.</p><p>In the low frequency region, the weak product absorption at698.7 cm-1 and its 13C counterpart at 694.0 cm-1 are assignedto the Ta-H in-plane bending mode without observation of theD counterpart. Another m absorption is observed at 657.9 cm-1</p><p>(with site absorption at 643.9 cm-1) along with its D counterpartat 508.7 cm-1 and 13C counterpart at 652.6 cm-1 (with a siteabsorption at 638.9 cm-1) (H/D and 12/13 ratios of 1.293 and1.008). The large D and small 13C shifts leads to an assignmentto the CH2 wagging mode. The m absorption at 464.9 cm-1 inthe 13CH313CN spectra is tentatively designated to the CH2rocking mode without observation of its 12C and D counterparts.</p><p>Observation of the Ta methylidene isocyanide is parallel tothe previously studied CH2dZr(H)NC,16 reconfirming that thesmall conjugated high oxidation-state complexes can be providedin reactions of transition-metal atoms with a nitrile compound.However, unlike the previous Zr + acetonitrile study, the Tainsertion and -complexes are not identified in the productspectra. The Ta methylidene isocyanide is the most stable amongthe plausible products. CH2dTa(H)NC, CH3-TaNC, 2-Ta(NC)-CH3, and CH3CNfTa in the doublet, quartet, doublet,and quartet ground states are 56, 48, 37, and 21 kcal/mol morestable than the reactants (Ta(4F) + CH3CN), respectively, andthe possible cyanide complexes, CH2dTa(H)CN(D) andCH3-TaCN(Q), are 52 and 47 kcal/mol more stable than thereactants.</p><p>Although the stability of CH2dTa(H)NC over other plausibleproducts is consistent with its exclusive generation,26 the absenceof CH2dTa(H)CN despite the comparable energy supports thepreviously proposed reaction path for the Zr system.</p><p>M + CH3CN f CH3CNfM f 2-(C)-CH3 f</p><p>CH3-MNC f CH2dM(H)NC (1)</p><p>Initial coordination of the metal atom to the electron-richN-end of acetonitrile is expected to form CH3CNfTa.27</p><p>Subsequent rearrangement will lead to the observed more stablenitrile -complexes28 in the Nb and V systems (described below)and the previous Zr study. The observed isocyanide insertionproducts from the Nb, V, and Zr reactions with no trace of thecyanide counterparts, which are energetically comparable,indicates that during C-C bond insertion by the metal atom,</p><p>the N-Ta bond is preserved, leading to the C-Ta-N-Cbackbone. The most stable CH2dTa(H)NC is produced in theend via H migration.</p><p>Nb + Acetonitrile. The product absorptions from Nbreactions with CH3CN isotopomers are shown in Figures 4 and5. In contrast to the Ta system, three groups of productabsorptions are observed depending on their intensity variationupon photolysis and annealing, which are marked m, i, and (for methylidene, insertion, and -complexes). The m absorp-tions are almost invisible in the original spectrum after codepo-sition, remain as weak on visible irradiation. They emerge onUV photolysis and almost double on full arc ( &gt; 220 nm)irradiation. The i absorptions decrease 30% and disappear onvisible and UV irradiations. The absorptions slightly increase,halve, and further decrease on visible, UV, and full arcirradiations, respectively. The observed intensity variations ofthe product absorption groups suggest that the products respon-sible for the i and absorptions convert to another productresponsible for the m absorptions in the process of photolysis.</p><p>The m absorption at 2039.9 cm-1 (with site absorptions at2035.2 and 2031.0 cm-1) in the CD3CN spectra in Figure 5 hasits 13C counterpart at 1999.4 cm-1 (with site absorptions at1994.8 and 1990.8 cm-1). On the basis of the frequency andsignificant 13C shift, it is assigned to the NC stretching mode.24d</p><p>The H counterpart expected...</p></li></ul>