Abstract
A series of dinuclear copper(I) N,C,N- and P,C,P-carbodiphosphorane (CDP) complexes using multidentate ligands CDP(Py)2 (1) and (CDP(CH2PPh2)2 (13) have been isolated and characterized. Detailed structural information was gained by single-crystal XRD analyses of nine representative examples. The common structural motive is the central double ylidic carbon atom with its characteristic two lone pairs involved in the binding of two geminal L-Cu(I) fragments at Cu–Cu distances in the range 2.55–2.67 Å. In order to enhance conformational rigidity within the characteristic Cu–C–Cu triangle, two types of chelating side arms were symmetrically attached to each phosphorus atom: two 2-pyridyl functions in ligand CDP(Py)2 (1) and its dinuclear copper complexes 2–9 and 11, as well as two diphenylphosphinomethylene functions in ligand CDP(CH2PPh2)2 (13) and its di- and mononuclear complexes 14–18. Neutral complexes were typically obtained via the reaction of 1 with Cu(I) species CuCl, CuI, and CuSPh or via the salt elimination reaction of [(CuCl)2(CDP(Py)2] (2) with sodium carbazolate. Cationic Cu(I) complexes were prepared upon treating 1 with two equivalents of [Cu(NCMe)4]PF6, followed by the addition of either two equivalents of an aryl phosphine (PPh3, P(C6H4OMe)3) or one equivalent of bisphosphine ligands bis[(2-diphenylphosphino)phenyl] ether (DPEPhos), 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (XantPhos), or 1,1′-bis(diphenyl-phosphino) ferrocene (dppf). For the first time, carbodiphosphorane CDP(CH2PPh2)2 (13) could be isolated upon treating its precursor [CH(dppm)2]Cl (12) with NaNH2 in liquid NH3. A protonated and a deprotonated derivative of ligand 13 were prepared, and their coordination was compared to neutral CDP ligand 13. NMR analysis and DFT calculations reveal that the most stable tautomer of 13 does not show a CDP (or carbone) structure in its uncoordinated base form. For most of the prepared complexes, photoluminescence upon irradiation with UV light at room temperature was observed. Quantum yields (ΦPL) were determined to be 36% for dicationic [(CuPPh3)2(CDP(Py)2)](PF6)2 (4) and 60% for neutral [(CuSPh)2(CDP(CH2PPh2)2] (16).
Highlights
In 1961, hexaphenyl-carbodiphosphorane, the first carbodiphosphorane (CDP), was synthesized by Ramirez et al [1]
To a classical ylide valence bond description, the bonding in carbodiphosphoranes can be decribed as a formal carbon(0) atom stabilized by two dative phosphine ligands with C–P retro dative bonding components, which is a model discussed earlier but quantified by a theoretical approach of Frenking and co-workers [2,3,4,5,6]
The central carbon atom is best described in its excited singlet (1 D) state [7]
Summary
In 1961, hexaphenyl-carbodiphosphorane, the first carbodiphosphorane (CDP), was synthesized by Ramirez et al [1]. The two characteristic occupied lone pairs (HOMO and HOMO+1) centered at this carbon atom ( named “carbone”) are either capable of binding two metals via two σ bonds in a close to tetrahedral configuration P2 CM2 or one metal in a trigonal–planar P2 CM configuration via a σ- and a π dative bond of very strong π,σ-donor character [8] For this reason, the coordination chemistry of carbodiphosphoranes has experienced a renaissance [9,10,11]. Efficient up-intersystem crossing (T1 →S1 ), designated as reverse intersystem crossing RISC, can occur at near ambient temperature [28,41,45,59,60], resulting in thermally activated delayed fluorescence (TADF) This is related to a small transition dipole moment, and a small radiative rate kr (S1 →S0 ) [31,32]. That high emission quantum yields can be obtained with selected materials of this class
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