Bimetallic, Spirocyclic, Methylene-Bridged Carbene Complexes of Rhodium and Palladium Derived by Stepwise Metalations of Lithiated Bis(diphenylphosphoranotrimethylsilylimido)methandiide

Abstract

The Li-C-Rh, bimetallic, phosphinimine spirocyclic, bridged carbene complex 1 was readily prepared from dilithiobis(diphenylphosphoranosilylimido)methandiide, [Li 2 L] 2 , and an equimolar (Rh/Li, 1:1) amount of [Rh(cod)Cl] 2 . Subsequent reaction with a different metal precursor, in the present cases, [Pd(allyl)Cl] 2 , yielded the spirocyclic Rh-Pd heterobimetallic 2 or, if a second equivalent of [Rh(cod)Cl] 2 was used, the spirocyclic Rh-Rh homobimetallic 3 was obtained; thus the system presents an effective synthetic route for the preparation of homo-or heterobimetallic complexes. All of the complexes have constrained symmetry, which creates in some cases isomeric and enantiomeric structures. The Li-Rh complex 1 thermally rearranged to the chelated, orthometalated, Rh complex 6, wherein the Rh(I) center is bound to the ipso carbon of a phenyl ring and a methine carbon formed from protonation of the spiro carbon. Remarkably, controlled hydrolysis of 6 reversed the orthometalation and provided an iminophosphorane-methine chelate complex of Rh(I), 7. Reactions of the cod-substituted complexes 2, 3, and 7 with CO resulted in the replacement of cod substituents to form parallel Rh carbonyl complexes (4, 5, and 8). The allyl substituents in the system were not affected. The carbonyl complexes 5 and 8 could be made independently from [Rh(CO) 2 Cl] 2 and either the dilithium methandiide or the methine form of the ligand as appropriate. Complexes 2-7 have been structurally characterized herein. Complexes 3 and 5 are chiral (because they lack rotation-reflection axes), but the products here are unresolved racemic mixtures according their NMR spectra. The complex 2 and its derivative 4 exist as unequal diastereomeric pairs according to the NMR. Fluxional NMR behavior is observed for 1, wherein the cod appears to partly dissociate and invert, for 2 and 3 where the cod exchange appears to involve a different mechanism being assisted by dissociation of one imine substituent, for 2 and 4 wherein allyl syn-syn, anti-anti exchange occurs between the diastereomers, and for 7 wherein the coordinated and uncoordinated imine groups exchange.

Synthesis and Structural Characterization of Mono‐ and Bimetallic Rhodium(I), Iridium(I) and Gold(I) Methanide and Methandiide Complexes from 2‐Quinolyl‐Linked (Thiophosphoranyl)methane

“…The Rh1–C10 and Rh1–S1 bond lengths of 2.198(2) and 2.4010(7) Å are comparable to [Rh(COD)CH(Ph 2 P=S)(SiMe 3 )] [Rh–C methanide 2.171(6), Rh–S 2.415(2) Å], [{Rh(COD)} 2 C(Ph 2 P=NSiMe 3 ) 2 ] (average Rh–C methandiide 2.158 Å) and other reported Rh pincer complexes . The Rh2–N1 bond length of 2.1409(17) Å is also comparable to [{Rh(COD)} 2 C(Ph 2 P=NSiMe 3 ) 2 ] (average Rh–N 2.189 Å) and other related Rh I complexes . Each Rh I center in 4 features a slightly distorted square‐planar geometry, which is commonly observed in 16‐electron complexes with d 8 ions.…”

“…Bimetallic complexes are defined as compounds containing two directly bonded or proximal non‐bonded metal atoms. Cavell et al have reported the use of a Li–C–Rh bimetallic phosphinimine‐bridged carbene complex to prepare other heterobimetallic complexes . It is well known that homo‐ or heterobimetallic complexes may exhibit different bonding modes, compared with monometallic complexes.…”

Geminal Dianions Stabilized by Main Group Elements

“…In a subsequent study, the authors showed that the Rh−Li heterobimetallic complex 44 could be used to form Rh−Rh homo or Rh−Pd heterobimetallic complexes 47 and 48 respectively. 136 In 2017, Gessner reported the synthesis of the Ir(III) carbene complex [Ir(11)(Cp*)]. Like for the related Ru(Cymene) complex, the carbene ligand is only bound via the thiophosphoryl moiety.…”

“…The final complex, 46 , features a rare bridging ketene ligand. In a subsequent study, the authors showed that the Rh–Li heterobimetallic complex 44 could be used to form Rh–Rh homo or Rh–Pd heterobimetallic complexes 47 and 48 respectively …”

Heterobimetallic Pd–K carbene complexes via one-electron reductions of palladium radical carbenes

“… 13 – 17 In organic synthesis, Shibasaki's rare-earth alkali-metal heterobimetallic complexes 18 are among the most enantioselective and broadly used catalysts to date, 19 – 21 but few studies discuss the role of Lewis acids in applications of late transition metals, in general, and of carbene complexes, in particular. 22 – 24 …”