Vibrational Spectroscopy Reveals Varying Structural Motifs in Cu⁺(CH₄)_n and Ag⁺(CH₄)_n (n = 1–6)

Abstract

Vibrational spectra are measured for Cu(+)(CH4)(Ar)2, Cu(+)(CH4)2(Ar), Cu(+)(CH4)n (n = 3-6), and Ag(+)(CH4)n (n = 1-6) in the C-H stretching region (2500-3100 cm(-1)) using photofragment spectroscopy. Spectra are obtained by monitoring loss of Ar or CH4. Interaction with the metal ion produces substantial red shifts in the C-H stretches of proximate hydrogens. The magnitude of the shift reflects the metal-methane distance and the coordination to the metal ion of the methane hydrogens (η(2) or η(3)). The structures of the complexes are determined by comparing the measured spectra with spectra calculated for candidate geometries using the B3LYP and CAM-B3LYP density functionals with 6-311++G(3df,3pd) and aug-cc-pVTZ-PP basis sets. Because of the d(10) electronic configuration of the metal ions, the complexes are expected to adopt symmetric structures, which is confirmed by the experiments. All of the complexes have η(2) hydrogen coordination in the first shell, in accord with theoretical predictions; second-shell ligands sometimes show η(3) hydrogen coordination. The vibrational spectrum of Cu(+)(CH4)(Ar)2 shows extensive structure due to Fermi resonance between the lowest-frequency C-H stretch and overtones of the H-C-H bends. The Cu(+)(CH4) cluster has a smaller red shift in the lowest-frequency C-H stretch than M(+)(CH4), M(+) = Co(+) (d(8)) and Ni(+) (d(9)). Although all three ions have similar binding energies, the metal-ligand electrostatic interaction is largest for Cu(+), while the contribution from covalent interactions is largest for Co(+). The larger ionic radius of Ag(+) leads to a larger metal-ligand distance and weaker interaction, resulting in substantially smaller red shifts than in the Cu(+) complexes. The Cu(+)(CH4)2 and Ag(+)(CH4)2 clusters have symmetrical structures, with the methanes on opposite sides of the metal, while Cu(+)(CH4)3 and Ag(+)(CH4)3 adopt symmetrical, trigonal planar structures with all M-C distances equal. For Cu(+)(CH4)4, the tetrahedral structure dominates the observed spectrum, although a trigonal pyramidal structure may contribute; however, only the tetrahedral structure is observed for Ag(+)(CH4)4. The structures of Cu(+)(CH4)n and Ag(+)(CH4)n differ for clusters with n > 4. For copper complexes, these are primarily formed by adding outer-shell methane ligand(s) to the tetrahedral n = 4 core. The observed spectra of the larger Ag(+) clusters are dominated by symmetrical structures in which all of the Ag-C distances are similar: Ag(+)(CH4)5 has a trigonal bipyramidal geometry and Ag(+)(CH4)6 is octahedral.

Infrared Spectroscopy of Au+(CH4) n Complexes and Vibrationally-Enhanced C–H Activation Reactions

“…Qualitatively similar features within the same IR range have also been observed in the IR depletion spectra of the other d 10 metal cation complexes, Cu + (CH 4 ) 4−6 and Ag + (CH 4 ) 5−6 recorded previously by Metz et al [ 78 ]. However, for n = 3 the IR depletion spectra presented here for the gold complexes differ significantly from those of the other coinage metals indicating a markedly different binding motif.…”

“…The Fe + , Co + and Ni + ions, with their 3 d n ground state configurations, interact with methane more strongly than ions with 3 d n −1 4 s 1 configurations, such as Mn + (CH 4 ) 1−6 . IR photofragmentation spectroscopy studies have also been performed on the d 10 complexes Cu + (CH 4 ) 1−6 and Ag + (CH 4 ) 1−6 [ 78 ], which tend to possess highly-symmetrical structures due to the spherical nature of the ion. The latter studies inspired the work presented here as part of our own development of a metal–ligand complex infrared dissociation instrument.…”

Lanthanum-mediated dehydrogenation of butenes: Spectroscopy and formation of La(C4H6) isomers

“…Spectroscopy of transition metal-hydrocarbon radicals or ions formed in gas phase reactions has recently attracted considerable attention. Metal ion-hydrocarbon species are largely investigated by infrared or ultraviolet-visible photodissociation or photoelectron spectroscopy, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] whereas metal atomhydrocarbon radicals are mainly studied by resonant twophoton ionization and dispersed fluorescence, [21][22][23][24] Fourier transform microwave, 25 and mass-analyzed threshold ionization (MATI) spectroscopy. [26][27][28][29][30][31][32][33] Spectroscopic measurements probe the state specific energetics and structures of shortlived species, which are vital for gaining insight into reaction mechanisms and electronic and structural characteristics for efficient bond activation at metal centers.…”

Spectroscopy and formation of lanthanum-hydrocarbon radicals formed by C—C bond cleavage and coupling of propene

“…Therefore, although a large body of thermodynamic and kinetic data is now available in the literature for gas-phase metal-hydrocarbon reactions as shown by numerous reviews, [6][7][8][9][10][11][12][13][14][15][16] spectroscopic measurements of metal-hydrocarbon species are lagged behind. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] As one of the simplest alkene molecules and the most important raw chemicals in petrochemical industry, metal-mediated propene activation has received considerable attentions. [6][7][8][9][10]14,15 For rare earth metal-propene reactions, previous studies have been reported on the product distributions and kinetics of both neutral atoms and positively charged ions.…”