The coordination chemistry of 2,4,6-oxy functionalised 1,3,5-triphosphinines

A number of stable group 6 metal complexes bearing 2,4,6-oxy functionalised 1,3,5-triphosphinines, phosphorus containing heterocyclic ligands with a central C₃P₃ core, were synthesised such that a complete series of [M{P₃C₃(OX)₃}(CO)₃] compounds is obtained [M = Cr(0), Mo(0), W(0); X = H, SitBuPh₂, B(ipc)₂]. In all complexes, the triphosphinine coordinates in a η⁶-binding mode via the delocalized 6π-system of the ring. The ligand properties can be tuned by changing the substituent on the oxygen centre. The π-electron accepting properties of the ligand increases in the following order: P₃C₃(OH)₃ < P₃C₃(OSi^tBuPh₂)₃ < P₃C₃(OB(ipc)₂)₃. This trend is reflected in the structures determined by X-ray crystallography, and the ν(CO) stretching frequencies determined by IR spectroscopy. The collected data raise questions with respect to the frequently made assumption that phosphinines act as stronger π-acceptors with respect to arenes and thereby deplete electron density at the metal centres. With P₃C₃(OH)₃ as an η⁶-coordinated ligand further molecules can be coordinated in the second coordination sphere via hydrogen bonds, which may be of interest for the construction of coordination polymers.

Highly flexible phosphabenzenes: a missing coordination mode of 2,4,6-triaryl-λ3-phosphinines

The reaction of 2,4,6-triaryl-λ³-phosphinine-Cr(CO)₃-π-complexes with [Rh(COD)₂]BF₄ leads to unusual diamagnetic Rh⁰-dimers, which contain two phosphinine-π-complexes acting as a bridging 2e^--ligand towards the Rh₂(CO)₂ core. These compounds represent a missing coordination mode for the aromatic 6-membered phosphorus heterocycle.

Making the unconventional μ2-P bridging binding mode more conventional in phosphinine complexes

As compared to the normal η¹-P σ-complexes or η⁶-phosphinine π-complexes, the rare μ²-P bridging binding mode of phosphinines can be tuned by employing electron donating substitute.

Phosphinines, as aromatic heterocycles, usually engage in coordination as η¹-P σ-complexes or η⁶-phosphinine π-complexes. The μ²-P bridging coordination mode is rarely observed. With the aim to study the effect of different electronic configurations of phosphinines on the coordination modes, a series of anionic phosphinin-2-olates and neutral phosphinin-2-ols were prepared with moderate to high yield. Then the coordination chemistry of these two series was studied in detail towards coinage metals (Au(i) and Cu(i)). It is observed that the anionic phosphinin-2-olates possess a higher tendency to take a bridging position between two metal centers compared to the neutral phosphinin-2-ols. Based on these experimental findings bolstered by DFT calculations, some insight is gained on how the unconventional μ²-P phosphinine bridging coordination mode can be made more conventional and used for the synthesis of polynuclear complexes.

Half-Sandwich Complexes of an Extremely Electron-Donating, Redox-Active η6-Diborabenzene Ligand

The heteroarene 1,4-bis(CAAC)-1,4-diborabenzene (1; CAAC = cyclic (alkyl)(amino)carbene) reacts with [(MeCN)₃M(CO)₃] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η⁶-diborabenzene)M(CO)₃] (M = Cr (2), Mo (3), W (4)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand 1 is a remarkably strong electron donor. In particular, [(η⁶-arene)M(CO)₃] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes 2-4 revealed one reversible oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η⁴ binding mode. Treatment of 4 with lithium metal in THF led to identification of the paramagnetic complex [(1)W(CO)₃]Li·2THF (5). Compound 1 can also be reduced in the absence of a transition metal to its dianion 1^2-, which possesses a quinoid-type structure.

Accessing multimetallic complexes with a phosphorus(i) zwitterion

We present the synthesis of a zwitterionic triphosphenium molecule, ^tBu(C₅H₂)(PPh₂)₂P^I (L), which can act as a single- or multidentate ligand with group 6, 7, 8 and 9 metal carbonyl complexes. Group 6, [M(CO)₅L] complexes are formed under photolytic conditions, where the metal is bound at the P(i) center. In the case of Mo(CO)₆, the bimetallic complex [M(CO)₅LMo(CO)₃] is generated, which features bonds to both the phosphorus(i) center and the cyclopentadienyl moiety of the molecule. Interestingly, group 7 and 9 metal carbonyl dimers generate bimetallic complexes in the form [M₂(CO)_nL], where both metal centers are bound at the phosphorus(i) center. A detailed analysis of these molecules is provided, including X-ray diffraction, multinuclear NMR, infrared spectroscopy and computational investigations.