Ligand K-edge X-ray absorption spectroscopy (XAS) is an experimental method that can quantify variations in covalent bonds and electronic structure in coordination complexes. A review of ligand K-edge XAS with emphasis on known P K-edge XAS studies of metal phosphine complexes is reviewed, followed by a description of how this method was used to investigate how ligand and structural variations affect As-S and M-P bonding. These studies were aimed at understanding how changes in As-S bonds are implicated in As toxicity and remediation, and developing structural diphosphine models that predict how structural variations affect M-P covalency in catalytically-relevant metal complexes.
A series of arsenic(III) dithiocarbamate complexes, As(S2CNR2)3, where R2 = Et2, (CH2)5 and Ph2, were prepared and characterized in order to investigate As-S bonding as a function of structural and electronic variations of N substituents. The crystal structures of As(S2CN(CH2)5)3 and As(S2CNPh2)3 are described along with the heteroleptic complex As[S2CN(CH2)5]2I. The synthesis, structures, and characterization of three new phosphonium dithiocarbamates, [PPh4][S2CNR2], are also reported.
S K-edge XAS data were collected on the As[S2CNR2]3 and [PPh4][S2CNR2] complexes to determine how dithiocarbamate substituents attached to N affect S bonding in the presence and absence of As. The XAS results revealed that changing the orientation of the alkyl substituents (cis vs. trans) have little effect on As-S bonding. In contrast, aryl substituents have a pronounced effect on As-S bonding and electronic structure. As K-edge XAS spectra, DFT, and TDDDFT theory calculations were also conducted in support of the results of the S K-edge XAS studies.
The effects of coordination geometry, bite angle, and trans influence on M-P bonding were investigated by analyzing a series of Ni and Pd phosphine complexes using P K-edge XAS. Cl K-edge XAS data, along with DFT and TDDFT theory calculations, were used to support P K-edge XAS results. The spectroscopic data revealed M-P covalency (1) is greater for square planar complexes than tetrahedral complexes, (2) is not solely dependent on bite angle in diphosphine complexes, and (3) can be used to quantify trans influence effects on M-P and M-Cl bonding.
Boraguanidinates, which have the general formula R2NB(NR`)22-, have rarely been used as ligands because they are susceptible to decomposition during metalation reactions. To overcome this issue, the bicyclic boraguanidinate 1,8,10,9-triazaboradecalin (TBD) was prepared to prevent decomposition upon reaction with metals and other chemicals. The synthesis and characterization of (H2TBD)TiCl4 will be discussed. Further, Lee et al. reported the synthesis of a diphosphine called TBDPhos, as well as (TBDPhos)NiCl2 and (TBDPhos)PdCl2 complexes, which displayed cooperative ligand-centered reactivity with water, alcohols and fluorides. The synthesis, characterization, and reactivity studies of Cu and Rh TBDPhos complexes are described here for the first time.
While cooperative ligand-centered reactions with TBDPhos have been explored with a variety of metals and substrates, the effects of TBD ligand modifications on the reactivity is unknown. Efforts at synthesizing new triazaboracycloalkanes are described in an effort to study how ligand-centered reactivity changes in response to (1) modifying the bridgehead N-B bond, (2) changing the size of the chelating boracycle, and (3) and adding alkyl substituents to the TBD backbone.