N‐Heterocyclic Carbene–Phosphinidene Complexes of the Coinage Metals

Abstract

Coinage metal complexes of the N-heterocyclic carbene-phosphinidene adduct IPr⋅PPh (IPr=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by its reaction with CuCl, AgCl, and [(Me2 S)AuCl], which afforded the monometallic complexes [(IPr⋅PPh)MCl] (M=Cu, Ag, Au). The reaction with two equivalents of the metal halides gave bimetallic [(IPr⋅PPh)(MCl)2 ] (M=Cu, Au); the corresponding disilver complex could not be isolated. [(IPr⋅PPh)(CuOTf)2 ] was prepared by reaction with copper(I) trifluoromethanesulfonate. Treatment of [(IPr⋅PPh)(MCl)2 ] (M=Cu, Au) with Na(BAr(F) ) or AgSbF6 afforded the tetranuclear complexes [(IPr⋅PPh)2 M4 Cl2 ]X2 (X=BAr(F) or SbF6 ), which contain unusual eight-membered M4 Cl2 P2 rings with short cuprophilic or aurophilic contacts along the chlorine-bridged M⋅⋅⋅M axes. Complete chloride abstraction from [(IPr⋅PPh)(AuCl)2 ] was achieved with two equivalents of AgSbF6 in the presence of tetrahydrothiophene (THT) to form [(IPr⋅PPh){Au(THT)}2 ][SbF6 ]2 . The cationic tetra- and dinuclear complexes were used as catalysts for enyne cyclization and carbene transfer reactions.

Transition‐Metal Carbonyl Complexes and Electron‐Donating Properties of N‐Heterocyclic‐Carbene–Phosphinidene Adducts

“…The C carbene –P bond lengths range from 1.805(2) Å (in 4c ) to 1.819(3) Å (in 4b ) and are significantly longer than those in the free ligands [1.7510(16) Å for R = H, 1.7658(10) Å for R = Ph, 1.766(2) Å for R = Mes], in agreement with the expected decrease of the C carbene –P bond order upon metal complexation . The metal–phosphorus bond lengths in the pairs 4a / 4b and 5a / 5b are almost identical, whereas the steric demand of the mesityl substituent in 4c affords a slightly elongated W–P bond (Table ).…”

“…In the latter study, it was also shown that cationic digold complexes such as [(IPr · PPh){Au(tht)} 2 ][SbF 6 ] 2 (tht = tetrahydrothiophene) may serve as robust single‐source catalysts in cycloisomerization or carbene‐transfer reactions, and these were the first applications of carbene–phosphinidenes as ancillary ligands in homogeneous catalysis. Moreover, the isolation of the monometallic complexes [(IPr · PPh)MCl] (M = Cu, Ag, Au) indicates that the stepwise complexation of the phosphorus atom in IPr · PPh is possible . This ability was also demonstrated previously by Lavoie, who reported the ruthenium(II) benzylidene complex [{IMes · PPh}RuCl 2 (CHPh)(PPh 3 )] …”

“…In all cases, the formation of pentacarbonyl complexes is confirmed, and they display slightly distorted octahedral geometries around the metal atoms. Similarly to those of other monometallic carbene–phosphinidene complexes,, , the phosphorus atoms reside in rather acute trigonal‐pyramidal environments, as indicated by angle sums of 329.4, 341.8, and 330.0° in 4b , 4c , and 5b , respectively. The P–M–C4 angles involving the axial carbonyl group are close to linearity for the parent phosphinidene complexes 4a and 5a [175.53(17) and 175.63(9)°, respectively], whereas a stronger deviation is observed for the sterically more congested complexes [170.24(10)° in 4b , 163.39(6)° in 4c , and 170.45(4)° in 5b ].…”

“…The IR spectra of the rhodium(I), tungsten(0), and molybdenum(0) carbonyl complexes 2 , 4 , and 5 demonstrate clearly that the carbene–phosphinidene adducts 1a – 1c act as strong electron‐donating phosphorus ligands; substantially lower CO stretching frequencies are observed in comparison with those of complexes with other important ancillary ligands such as phosphines and N‐heterocyclic carbenes. In view of the potential application of carbene–phosphinidene ligands in homogeneous catalysis, it can be concluded that these systems are particularly electron‐rich additions to the large family of P‐based ligands, whereas related cationic NHC‐based phosphorus ligands such as phosphenium cations of the type [(NHC)PR 2 ] + were presented recently as electron‐poor and strongly π‐accepting ligands . The NHC skeleton and also the phosphinidene moiety in NHC · PR ligands can be varied extensively to fine‐tune the electronic and steric properties to adapt to the demands of a specific catalytic process.…”

“…As indicated above, carbene–phosphinidene adducts can be regarded as promising ancillary phosphorus ligands in homogeneous catalysis; therefore, an assessment of their electronic and steric properties is required. Transition‐metal carbonyl complexes can be used to study ligand donor properties through the monitoring of their CO stretching frequencies by IR spectroscopy.…”