Tweaking the Charge Transfer: Bonding Analysis of Bismuth(III) Complexes with a Flexidentate Phosphane Ligand

To account for the charge transfer and covalent character in bonding between P and Bi centers, the electronic structures of [P(C6H4-o-CH2SCH3)3BiCln](3–n)+ (n = 0–3) model species have been investigated computationally. On the basis of this survey a synthetic target compound with a dative P→Bi bond has been selected. Consecutively, the highly reactive bismuth cage [P(C6H4-o-CH2SCH3)3Bi]3+ has been accessed experimentally and characterized. Importantly, our experiments (single-crystal X-ray diffraction and solid-state NMR spectroscopy) and computations (NBO and AIM analysis) reveal that the P···Bi bonding in this trication can be described as a dative bond. Here we have shown that our accordion-like molecular framework allows for tuning of the interaction between P and Bi centers.


Materials and methods
All the reactions reported here were performed using standard Schlenk techniques under a dry nitrogen atmosphere. Sensitive chemicals were stored and weighed in a glove box under nitrogen atmosphere. All the solvents employed were purified and dried by standard methods.
Tri(o-meththiomethylphenyl)phosphine P(C 6 H 4 -o-CH 2 SCH 3 ) 3 (PS 3 ) was prepared according to a previously published protocol. 1 The water content of commercially available bismuth triflate was determined by thermogravimetric analysis and the compound has a composition of Bi[OTf] 3 (H 2 O) n (n ≈ 14). Bismuth triflate used for the synthesis of [PS 3 Bi] 2 [BOT] was previously dried for 10 days at 160°C in a dynamic vacuum to obtain [Bi(OTf) 3 (H 2 O) 2.8 ] (water content determined by elemental analysis). This agrees with previous reports highlighting the (to date) sheer impossibility to synthesize strictly anhydrous Bi(OTf) 3 . 2 Solution NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. Chemical shifts are reported in ppm relative to SiMe 4 ( 1 H, 13 C), CFCl 3 ( 19 F) and 85% H 3 PO 4 ( 31 P).
Coupling constants are given in Hz. Solid-state NMR spectra were recorded on a 400 MHz Bruker Avance III HD spectrometer. Elemental analyses were performed at the Science Centre at London Metropolitan University. X-ray diffraction experiments were carried out at T=120 K on a Bruker 3-circle D8 Venture diffractometer with a PHOTON 100 CMOS area detector, using Mo-K α radiation (λ=0.71073 Å) from an Incoatec IμS microsource with focussing mirrors and a Cryostream (Oxford Cryosystems) open-flow N 2 gas cryostat. The structures were solved by direct methods (SHELXS) 3 and refined by full-matrix least squares using SHELXL software 4 on OLEX2 platform. 5

On the refinement of [PS 3 Bi] 2 [BOT]
The asymmetric unit contains two MeCN molecules in general positions (i.e. not having crystallographic symmetry). Of these, molecule N(1)CMe is incompatible with one of the two alternative conformations of the disordered CH2-S-Me chain, as this would result in an impossibly short contact S(1A)…N(1). However, the actual degree of the disorder is not rigorously imposed by crystal symmetry and may vary. The molecule N(2)CMe can, in principle, have a stoichiometric (100%) occupancy, but the actual electron density is lower than this. The 50% occupancies of both acetonitrile molecules, as well as S(1A), give the best fit to the observed X-ray data. This corresponds to six MeCN molecules per formula unit. The water molecule is disordered between three positions (with optimised occupancy of 1/6) slightly offset from a 3-fold rotation axis and related by this axis. Because of this disorder, it was impossible to locate the hydrogen atoms reliably, so the latter were not included into the refined model -however, they were included in the formula and taken into account in all formula-related parameters (f.w., F000, density). The use of restraints was necessitated by extensive disorder of the structure. The bond distances in the two alternative positions of each disordered moiety were restrained to similarity (SADI). The atomic displacement parameters in the disordered arene ring and part-occupied MeCN molecules were restrained using rigidbond model (RIGU). 6 Regarding the B-alerts in the Cif-file: 1) Much of the molecule is disordered and some of the disordered F, O and C atoms were refined in isotropic approximation, anisotropic refinement proving unstable due to small distances between alternative positions of these atoms.
2) The structure contains voids with the nominal volume of 158 A3, however, the awkward shape of these voids (as thin fragments of spherical shells) make them unsuitable to accommodate solvent molecules. No substantial residual electron density was detected in these voids.

Computational details
The computations were carried out with the Gaussian 09 suite of programs. 7 All structures were optimised using the ωB97XD and B3LYPD3 functionals. We employed the all valence cc-pVDZ basis set for H, C, P, S, Cl, O, and F while for Bi the cc-pVDZ-PP basis set including pseudopotentials for the modelling of relativistic effects was used. The basis set with pseudo potentials was obtained from the EMSL Basis Set Library (https://www.basissetexchange.org/). [8][9] At each of the optimised structures vibrational analysis was performed to check whether the stationary point located is a minimum or a saddle point of the potential energy hypersurface. For Wiberg Bond Indices and NPA charges the NBO program version 3.1 was used. 10 The AIM analysis was obtained with the Multiwfn code. 11 The calculation of indirect spin-spin coupling constants was carried out with the ADF 2014 program 12-14 at the PBE1/TZ2P level with scalar ZORA approximation on geometries optimized at ωB97XD/cc-pVDZ-PP.