Heterodinuclear Cu(i)/Mo(vi) chemistry with bifunctional dibenzobarrelene ligands

Bifunctional dibenzobarrelene ligands combining diphosphine with diol or other potentially bidentate functionalities are well-known mononucleating ligands for middle and late transition metals. These ligands generally exhibit tetradentate coordination, in which a metal is coordinated by [PC(sp³)P] 3-dimensional pincer and an additional alkoxide donor. These ligands can also be envisioned as heterodinucleating ligands, which coordinate a soft, late metal (via the diphospine) and a hard, early metal (via the diolate); the heterodinucleating reactivity of these ligands has not been studied. Herein we describe our initial studies on Cu(I)/Mo(VI) chemistry of the diphosphine/diol ligand L¹H₂; it is compared with the dibenzobarrelene ligand combining diphosphine with diester (L²). Both ligands led to stable Cu(I) complexes in which the ligand coordinates as a bidentate diphosphine. Spectroscopic characterization (carried out in CD₂Cl₂) revealed an AB system for the diastereotopic phosphine groups in ³¹P {¹H} NMR spectrum. The phosphorus signals appear as an AB system due to the large values of ²J_P–P, 120–150 Hz. ⁶³Cu NMR spectroscopy (conducted in concentrated CD₃CN solutions) revealed broad Cu signals at, or below, room temperature. The observation of a quadrupolar ⁶³Cu (I = 3/2) signal was accompanied by the broadening of the ³¹P signals, which now appear as a single broad peak. The reaction of both ligands with (Et₄N)₂[MoO₄] failed to form well-defined products. Treatment of [Cu(NCMe)₂(L¹H₂)](PF₆) with (Et₄N)₂[MoO₄], followed by recrystallization from DMSO/ether, revealed the structure of a trinuclear complex [MoO₂(μ₂-O)₂{Cu(DMSO)(L¹H₂)}₂] in which one Mo(VI) center is linked with two Cu(I) centers via oxo bridging ligands. In DMSO solution, the trinuclear complex likely forms discrete ionic species [Cu (DMSO)₂(L¹H₂)]₂[MoO₄] or [Cu(DMSO)₂(L¹H₂)][Et₄N][MoO₄].