Formation of Short Zn−Zn Bonds Stabilized by Simple Cyanide and Isocyanide Ligands

Abstract

Cyanogen diluted in argon was reacted with laser ablated Zn atoms to produce the NCZnCN and NCZnZnCN cyanides and higher energy isocyanides ZnNC, CNZnNC, and CNZnZnNC, which were isolated in excess argon at 4 K. These reaction products, identified from the matrix infrared spectra of their ‐CN and ‐NC chromophore ligand stretching modes, were confirmed by 13C and 15N isotopic substitution and comparison with frequencies calculated by the B3LYP and CCSD(T) methods using the all electron aug‐cc‐pVTZ basis sets. The cyanide and isocyanide products were increased markedly by mercury arc UV photolysis, which covers the zinc atomic absorption. The above electronic structure calculations that produce appropriate ligand frequencies for these dizinc products also provide their Zn−Zn bond lengths: CCSD(T) calculations find a short 2.367 Å Zn−Zn bond in the NCZnZnCN cyanide, a shorter 2.347 Å Zn−Zn bond in the 37.4 kJ mol−1 higher energy isocyanide CNZnZnNC, and a longer 4.024 Å bond in the dizinc van der Waals dimer. Thus, the diatomic cyanide (‐CN) and isocyanide (‐NC) ligands are as capable of stabilizing the Zn−Zn bond as many much larger ligands based on their measured and our calculated Zn−Zn bond lengths. This is the first example of dizinc complexes stabilized by different ligand isomers. Additional weaker bands in this region can be assigned to the analogous trizinc molecules NCZnZnZnCN and CNZnZnZnNC.

Cyanides and Isocyanides of Zinc, Cadmium and Mercury: Matrix Infrared Spectra and Electronic Structure Calculations for the Linear MNC, NCMCN, CNMNC, NCMMCN, and CNMMNC Molecules

“…A recent examination of the laser ablated Zn atom reaction with NC=CN in excess argon during condensation at 4 K produced ZnNC at 2083.6 cm −1 as the strongest sharp new absorption in these experiments, [30] and this identification was supported by 13 C and 15 N substitution in the cyanogen precursor and product vibrational frequency calculations [30] . The Gaussian‐16 system with the CCSD(T)/aug‐cc‐pVTZ [19–23] methods predicted the Zn−N−C stretching fundamental at 2092.6 cm −1 , exactly 9.0 cm −1 higher than the argon matrix observation at 2074.6 cm −1 , [24] which is in excellent agreement since much of this 9.0 cm −1 difference could be due to anharmonicity and/or the expected red matrix shift.…”

“…The absorptions observed for ZnNC, NCZnCN, CNZnNC, NCZnZnCN, and CNZnZnNC set the stage for identification of the Cd counterparts for these Zn products [30] . Indeed the Cd bearing molecule strongest absorptions were observed here 4.8, 5.9, 4.8, 8.7 and 5.5 cm −1 lower, respectively, than the above Zn counterparts.…”

“…The products from similar reactions with Group 2 metal atoms will also be compared, since the radiation supplied by different metals will be different [22] . The strongest bands of the analogous zinc, cadmium and mercury products as well as the dizinc, dicadmium and dimercury cyanides and isocyanides follow the example of diberyllium in gaining Be−Be bond strength through anti‐bonding pi electron withdrawal by electronegative substituents [23,25,30] …”

“…Photolysis of cyanogen by this ablation plume with Vis, UV, and VUV light directed at the sample form a distance of 5 cm is a very effective irradiation source for the matrix sample during co‐deposition with the metal atoms in excess argon, which also forms both the asymmetric CNCN and the symmetrical CNNC isomers, [21] in addition to the direct metal insertion CNMNC and NCMCN reaction products. We have recently investigated reactions of laser ablated Hg and Zn atoms with (CN) 2 [23,30] . These apparently involve stepwise relaxation of the activated intermediate [NC−M−CN]* first formed in the matrix.…”

“…The beryllium dimer itself has been characterized as a weakly bonded van der Waals molecule with a bond dissociation energy of 10 kJ/mol [23,25,30] . Recently the electronic structure of Be 2 complexes with radical ligands (L : Be−Be : L) have been calculated using CCSD(T)/aug‐cc‐pVTZ theory, and substantial increases in the Be−Be bond strength have been predicted [23] .…”