A Spirocyclic, Bimetallic, Carbon-Bridged Bis(iminophosphorano)methanediide Complex of Palladium(II): Synthesis, Crystal Structure, Solution Behavior, and DFT Bonding Analysis

Abstract

The dimeric dilithium methanediide salt [Li 2 C(Ph 2 PdNSiMe 3 ) 2 ] 2 ([Li 2 -1] 2 ) reacted with either 1 or 2 equiv of [PdCl(allyl)] 2 (allyl = CH 2 dCHCH 2 ) in toluene or benzene to give exclusively the bimetallic Pd spirocyclic carbon-bridged complex [{η 3 -C 3 H 5 )Pd} 2 {μ,κ 2 :κ 2 -C(Ph 2 PdNSiMe 3 ) 2 }] (2). Complex 2 was fully characterized by solution multinuclear NMR ( 1 H, 31 P, and 13 C) spectroscopy in toluene-d 8 and by X-ray crystallography. NMR reveals three coexisting isomers (labeled as: 2I 1 , 2I 2 , and 2I 3 ) in the molar ratios 2/1.73/0.45 in solution. These isomers are attributed to different relative orientations of the allyl groups on Pd. The crystal and solid-state structure similarly shows the visible presence of isomers with differing allyl orientations. The isomer ratio in the solid is, however, not the same as in the solution. The minor isomer in the solid, 2I 1 , has an asymmetrical molecular configuration which would create inequivalent phosphorus atoms, whereas the symmetric isomeric forms (which comprise the major proportion of the complex in the crystal) display geometries with a pseudo 2-fold axis relating the two phosphorus centers, thereby creating NMR equivalence. This is consistent with the appearance in solution of one asymmetric isomer with inequivalent phosphorus atoms and two symmetric isomers with equivalent phosphorus centers. Variable-temperature NMR spectra showed that each of the three isomers present in solution suffers conformational exchange; however, full exchange was not observed in the 31 P NMR-the different isomeric signals were clearly detectable up to 100 °C and were still separated at 115 °C. There is an apparent shift in isomeric distributions with temperature, but cooling the sample back to room temperature reestablishes the initially observed ratios of the three isomers. No thermal decomposition was observed in the VT NMR studies or in specific heating experiments to 115 °C. Under oxygen-and moisture-free conditions, 2 is remarkably inert; the allyl or the bridging carbon atom (which can be classified as a bridging carbene) are not displaced by CO. Interestingly, the 13 C NMR chemical shift of the bridged Pd-C(carbene) appeared at a high field (-7.0 to -10.0 ppm), which indicates an electron-rich PCP carbene presence in 2. This appears to manifest its presence by relatively long Pd-C(carbene) bond lengths and shorter Pd-C(allyl) bond lengths. Gaussian DFT calculation and NBO analysis revealed a net back-donation from the Pd to the C carbene along with a delocalization throughout the backbone skeleton which stabilizes the molecular structure. Scheme 1. Pincer, Chelate, and Spirocyclic Complexes of the Bis(iminophosphorano)methanediide Ligand