Low Temperature Isolation of a Dinuclear Silver Complex of the Cyclotetraphosphane [clp(µ-pmes*)] 2 †

a The reaction of the cyclotetraphosphane [ClP(µ-PMes*)] 2 (1, Mes* = 2,4,6-tri-tert-butylphenyl) with Ag[Al(OR F) 4 ] (R F = CH(CF 3) 2) resulted in a labile, dinuclear silver complex of 1, which eliminates AgCl above −30 °C. Its properties were investigated by spectroscopic methods, single crystal X-ray diffraction and DFT calculations.


Molecular structure
Most interestingly, both chlorine atoms remained at the P 4 scaffold; nonetheless two rather long Ag⋯Cl contacts were observed, which are shorter than the sum of van der Waals radii (Ag1-Cl2A 3.496(1) Å, Ag1-Cl2A′ 3.641(2) Å; cf. ∑r vdW = 4.35 Å) 44 as revealed by single crystal X-ray diffraction (Fig. 1). The P-Ag bond lengths (P1A-Ag1 2.394(2) Å, P3a-Ag1′ 2.391(2) Å) compare well to the sum of the covalent radii (2.39 Å), 45 whereas the Ag-Ag distance (3.0511(7) Å) lies between the sum of the covalent (2.56 Å) 45 and van der Waals radii (5.06 Å). 44 The silver atoms are almost linearly coordinated (177.52(5)°) and lie in a perfect plane with the Mes* substituted P atoms. Strikingly, the configuration at P2 is inverted in comparison with the starting material, where both chlorine atoms are arranged in an equatorial position with respect to the P 4 ring system. The dinuclear silver complex is nicely shielded by all four Mes* substituents: the Ag atoms and the equatorial Cl atoms (Cl2A, Cl2A′) are protected by the ortho-tBu groups, while the axial Cl atoms (Cl1A, Cl1A′) are sandwiched between the phenyl rings ( Fig. S1, ESI †). A similar coordination pattern was previously observed in [(Cy 4 P 4 ) 2 Sb 2 Cl 2 ] 2+ (Cy = cyclohexyl), where a planar Sb 2 Cl 2 scaffold is coordinated by two Cy 4 P 4 rings. 46 To the best of our knowledge, it is the only other example of a dinuclear metal complex capped by two P 4 ring systems.

Spectroscopic characterization
The low temperature 31 P NMR spectrum (−60°C) of 5[Al(OR F ) 4 ] 2 showed a complex AA′BB′MM′M″M′′′XX′ spin system (Fig. 2) 47,48 Due to the complex multiplet structure and generally low solubility of 5[Al(OR F ) 4 ] 2 , the signal to noise ratio of the NMR spectrum was rather poor, which is why only the large 1 J coupling constants could be determined unambiguously, while all smaller coupling constants have higher uncertainties. Nonetheless, the experimental data agree well with calculated NMR shifts and coupling constants (Table S3 †

Computational study
To further investigate the bonding situation in 5 2+ , density functional theory (DFT) calculations were performed. † According to NBO analysis, 49 there is no significant bonding interaction between the two Ag atoms, as already indicated by the nearly linear coordination of the Ag centres. Furthermore, both Wiberg bond index (0.31) and natural bond index (0.56) indicate a low covalent character of the Ag-P bonds; accordingly, the natural Lewis representation comprises two distinguished Ag + cations and two neutral cyclophosphane moieties. Second order perturbation analysis reveals two stabilizing interactions per Ag + cation between the empty s-orbital and the lone pairs (LPs) of the flanking P atoms (346.2 kJ mol −1 each), which is consistent with a classical dative bond from P to Ag (Fig. 3). The natural partial charge of each Ag centre is +0.59e, while each of the four coordinating P atoms bears a charge of +0.17e. Hence, the formal charge transfer amounts to −0.41e per Ag + ion.

Intramolecular elimination of AgCl
When the reaction mixture of the cyclophosphane 1 and the silver salt 4 was allowed to warm to temperatures above −30°C, precipitation of a white solid was observed, indicating elimination of AgCl. In situ 31 P NMR spectroscopy revealed that the intermediately formed silver complex 5 2+ decomposed above that temperature (Fig. S3 †), yielding the bicyclic cation [Mes*P 4 (Cl)Mes*] + (3 + , Scheme 3), which had previously been obtained by reacting 1 with GaCl 3 (Scheme 2, top). 43

Conclusions
In conclusion, we present a thermally labile dinuclear silver complex capped by two P 4 ring systems, which eliminates AgCl at temperatures above −30°C. Thus it can be considered an intermediate of the chloride abstraction from [ClP(µ-PMes*)] 2 , demonstrating that the reaction occurs via an intramolecular rather than an intermolecular process. Fig. 3 Left: molecular orbitals (MOs) of 5 2+ . The HOMO shows contributions to the LPs at the coordinating P atoms, the HOMO−8 encompasses the LPs at Cl and the other P atoms as well as bonding within the P 4 ring systems. Right: natural bond orbital (NBO) representation of a LP at phosphorus and the corresponding natural localized molecular orbital (NLMO), which consists mainly of a formal sp hybrid orbital at P (86%) and an s orbital at Ag (11%), thus illustrating the bonding between these two centres.