Synthesis and Structural Studies of peri-Substituted Acenaphthenes with Tertiary Phosphine and Stibine Groups

Two mixed peri-substituted phosphine-chlorostibines, Acenap(PiPr2)(SbPhCl) and Acenap(PiPr2)(SbCl2) (Acenap = acenaphthene-5,6-diyl) reacted cleanly with Grignard reagents or nBuLi to give the corresponding tertiary phosphine-stibines Acenap(PiPr2)(SbRR’) (R, R’ = Me, iPr, nBu, Ph). In addition, the Pt(II) complex of the tertiary phosphine-stibine Acenap(PiPr2)(SbPh2) as well as the Mo(0) complex of Acenap(PiPr2)(SbMePh) were synthesised and characterised. Two of the phosphine-stibines and the two metal complexes were characterised by single-crystal X-ray diffraction. The peri-substituted species act as bidentate ligands through both P and Sb atoms, forming rather short Sb-metal bonds. The tertiary phosphine-stibines display through-space J(CP) couplings between the phosphorus atom and carbon atoms bonded directly to the Sb atom of up to 40 Hz. The sequestration of the P and Sb lone pairs results in much smaller corresponding J(CP) being observed in the metal complexes. QTAIM (Quantum Theory of Atoms in Molecules) and EDA-NOCV (Energy Decomposition Analysis employing Naturalised Orbitals for Chemical Valence) computational techniques were used to provide additional insight into a weak n(P)→σ*(Sb-C) intramolecular bonding interaction (pnictogen bond) in the phosphine-stibines.


Introduction
Tertiary amines and phosphines play a key role as tuneable ligands, with uses in transition metal catalysis and other applications.The heavier tertiary pnictines (ER 3 , E = As, Sb, Bi, R = alkyl, aryl) also serve as L-type ligands in a number of complexes, although they generally display lower donor strength than corresponding N and P-based ligands [1,2].
Several unusual properties stemming from the close-proximity of two pnictine groups in peri-substituted scaffolds have been noted.The first of these was in the 1960's, when the remarkably high basicity of proton sponge 1,8-bis(dimethylamino)naphthalene (A, Figure 1) was reported by Alder [3].Tertiary bis(phosphines) such as the phosphorus analogues of the proton sponge B (Figure 1) were reported shortly thereafter [4,5], as were several of their metal complexes [6][7][8].
Syntheses of peri-substituted tertiary bis(arsines), such as C (Figure 1), and their complexes were also reported as early as the 1960's [9].However, no crystal structures of such ligands or complexes have appeared in the literature.Prototypical naphthalene bis(stibines) with dimethylstibino (D1) and diphenylstibino groups (D2) were synthesised by Reid, together with their Mo(0) and Pt(II) complexes [10].The aryl species D2 (both naphthalene and acenaphthene variants) and D3 (naphthalene variant) were recently structurally characterised by Schulz, together with the two bis(bismuthines) E [11,12].However, no structural data for any of the bis(stibine) or bis(bismuthine) metal complexes have been published to date.Syntheses of peri-substituted tertiary bis(arsines), such as C (Figure 1), and their complexes were also reported as early as the 1960's [9].However, no crystal structures of such ligands or complexes have appeared in the literature.Prototypical naphthalene bis(stibines) with dimethylstibino (D1) and diphenylstibino groups (D2) were synthesised by Reid, together with their Mo(0) and Pt(II) complexes [10].The aryl species D2 (both naphthalene and acenaphthene variants) and D3 (naphthalene variant) were recently structurally characterised by Schulz, together with the two bis(bismuthines) E [11,12].However, no structural data for any of the bis(stibine) or bis(bismuthine) metal complexes have been published to date.
The related Sb−Sb bonded species F [12], as well as the doubly backboned species G [13][14][15] and H [11], have received significant attention recently, and a few transition metal complexes with these as ligands have also been reported [13].
Species I (Figure 1) with two differing Group 15 peri-atoms display intriguing dative bonding and NMR properties [16,17].Surprisingly, only two bis(tertiary) phosphine-stibine and phosphine-bismuthine peri-substituted species have been reported to date: ISb [18]  The related Sb−Sb bonded species F [12], as well as the doubly backboned species G [13][14][15] and H [11], have received significant attention recently, and a few transition metal complexes with these as ligands have also been reported [13].
Species I (Figure 1) with two differing Group 15 peri-atoms display intriguing dative bonding and NMR properties [16,17].Surprisingly, only two bis(tertiary) phosphine-stibine and phosphine-bismuthine peri-substituted species have been reported to date: I Sb [18] and I Bi [19].Both of these display repulsive interactions between the two pnictogencentred groups, although their geometries indicate a weak pnictogen bond (nP→σ*(Pn-C)) is present, as indicated in Figure 1 by a dashed line.This is in contrast to the related E(III)−E(III) halophosphines, such as J [18,20] and K [18,21], which display strong dative pnictogen-pnictogen bonds.
Apart from the phosphine-stibine I Sb [18], the most closely related work to this paper are the geminally substituted bis-and tris(acenaphthene) species L and M [22,23].Only one metal complex of these has been structurally characterised, the Rh(I) species N [23].
As a continuation of our synthetic, structural and bonding studies of peri-substituted species, we investigated the utility of the halostibines J and K (Figure 1), reported by us earlier [18], as synthons towards primary stibine functionalities.Prompted by the paucity of the literature data, we have also probed the coordination chemistry of the produced tertiary phosphine-stibines.

Results and Discussion
2.1.Synthesis and Spectroscopic Properties of the Tertiary Stibines 4, 5, 7, and 8 The three major precursors for the syntheses reported in this paper were bis(aryl) stibine 2, chloro(aryl) stibine 3 and dichlorostibine 6 (Scheme 1).Syntheses and structural information for these compounds, starting from 1, have been reported by us [18].The reactive Sb−Cl motifs in chlorostibine 3 and dichlorostibine 6 were used to form new Sb−C bonds via reactions with carbon nucleophiles.The chlorostibine 3 was reacted with one equivalent of alkyl-Grignard reagents, MeMgBr and iPrMgCl, to afford alkylaryl stibines 4 (86%) and 5 (81%), respectively.
The reaction of dichlorostibine 6 with two equivalents of MeMgBr afforded dimethylstibine 7 as an off-white solid (yield ca.90%; exact yield determination was not possible as 1 H NMR indicated solvation by Et2O).Reacting nBuLi with 6 also resulted in Sb−C bond formation; reaction with two equivalents of nBuLi afforded crude di-n-butylstibine 8 in quantitative yield (obtained as an oil).The crystallisation of 8 from common organic solvents was not successful.However, a small amount of crystalline 8 was obtained Scheme 1. Syntheses of the peri-substituted phosphine-stibines reported in this paper.Compounds highlighted in frames are newly synthesised here.Note: nbd = norbornadiene, cod = 1,5-cyclooctadiene.
The reaction of dichlorostibine 6 with two equivalents of MeMgBr afforded dimethylstibine 7 as an off-white solid (yield ca.90%; exact yield determination was not possible as 1 H NMR indicated solvation by Et 2 O).Reacting nBuLi with 6 also resulted in Sb−C bond formation; reaction with two equivalents of nBuLi afforded crude di-n-butylstibine 8 in quantitative yield (obtained as an oil).The crystallisation of 8 from common organic solvents was not successful.However, a small amount of crystalline 8 was obtained through the long standing of the oil at room temperature (see below).
All the Sb−C bond-forming reactions were remarkably clean, as judged by 31 P{ 1 H} and 1 H NMR spectroscopy.The newly prepared compounds were further characterised by 13 C DEPT-Q NMR, HRMS (peaks corresponding to (M + H) + with correct isotopic patterns were observed in all cases) and (for 7 and 8) also by Raman spectroscopy.The purity of 5 was confirmed by CHN microanalysis.The novel tertiary stibines appear to be hydrolytically stable (in some cases, an aqueous wash was involved in the work-up); however they are oxidised in the presence of air.Both 7 and 8 decomposed in chloroform solutions within several days, indicating instability in halogenated solvents.
The reaction of 6 with one equivalent of nBuLi gave an oil after the workup.This oil was shown by 31 P{ 1 H} NMR to be a complex mixture, with a major peak at δ P 18.5 ppm, corresponding to the doubly substituted species 8, indicating that selective single substitution using an organolithium as a nucleophile may be difficult to achieve.
The 31 P{ 1 H} NMR spectra of the phosphine-stibines 4, 5, 7 and 8 display singlets within a narrow range of δ P (−18.5 to −20.8 ppm).Notable through-space couplings (indicated by the TS superscript in the J notation, TS J) are observed in the 13 C{ 1 H} NMR spectra between the phosphorus atom and carbons attached to the antimony atom.In 4, the ipso-C of the Sb-Ph moiety shows a 5TS J CP of 16.2 Hz.An even larger 5TS J CP of 34.1 Hz is observed for the Sb-CH 3 of 4. Interestingly, the acenaphthene ipso-carbon atom shows no detectable coupling to the phosphorus atom, despite having a shorter bond path (formally 3 J CP ).
A similar situation is observed in 5 ( 5TS J CP = 17.3 Hz (ipso-Ph) and 5TS J CP = 36.8Hz (Sb-CH), although in this case small magnitude splitting with the ipso-acenaphthene carbon (C1 in the numbering scheme shown in Figure 2) is observable ( 3 J CP = 2.4 Hz).Similar magnitudes of J CP involving carbon atoms bonded directly to Sb atoms are also observed for 7 ( 5TS J CP = 34.9Hz (CH 3 ); 3 J CP = 5.1 Hz (C1, Acenap)) and 8 ( 5TS J CP = 30.6Hz (CH 2 ); 3 J CP = 5.0 Hz (C1, Acenap)).Observation of the through-space couplings in 4, 5, 7 and 8 is consistent with the significant overlap of P and Sb lone pairs as confirmed by single crystal X-ray diffraction (vide infra) and is in agreement with observations made in our previous study of P-Sb acenaphthenes [18].

Synthesis and Spectroscopic Properties of Tertiary Stibine Metal Complexes 2.PtCl 2 and 4.Mo(CO) 4
Peri-substituted species 2 and 4, bearing tertiary phosphine and tertiary stibine groups, were reacted with platinum(II) and molybdenum(0) motifs to explore their coordination chemistry.It was of interest to see if the phosphine-stibine species would act as bidentate ligands, with the metal coordinating through both phosphorus and antimony atoms.
[PtCl 2 (cod)] was reacted with 2 in dichloromethane, giving 2.PtCl 2 as a yellow powder in a good yield (76%).Similarly, the reaction of [Mo(CO) 4 (nbd)] with 4 in dichloromethane gave 4.Mo(CO) 4 as a brown powder in a near-quantitative yield (Scheme 1).Both complexes were stable to air in the solid and solution in the chlorinated solvents used to acquire their NMR spectra (CD 2 Cl 2 and CDCl 3 , respectively).

Synthesis and Spectroscopic Properties of Tertiary Stibine Metal Complexes 2.PtCl2 and 4.Mo(CO)4
Peri-substituted species 2 and 4, bearing tertiary phosphine and tertiary stibine groups, were reacted with platinum(II) and molybdenum(0) motifs to explore their coordination chemistry.It was of interest to see if the phosphine-stibine species would act as The 31 P{ 1 H} NMR spectrum of 2.PtCl 2 consists of a singlet with a set of 195 Pt satellites (δ P 7.8 ppm, 1 J PPt = 3357 Hz), with the complementary doublet observed in the 195 Pt{ 1 H} NMR spectrum (δ Pt −4541 ppm).The coordination of platinum centres resulted in a highfrequency shift (c.f.free ligand 2, δ P −21.9 ppm) [18] as well as loss of the through-space J CP coupling (c.f.5TS J CP 40.3 Hz for ipso-Ph carbon in 2).
Coordination of the Mo(CO) 4 fragment to 4 resulted in an even more pronounced high-frequency shift for 4.Mo(CO) 4 (δ P 43.4; c.f. δ P −19.6 ppm in free ligand 4).Similar to 2.PtCl 2 , the J CP couplings between the phosphorus atom and the carbon atoms adjacent to the antimony atom are much smaller magnitudein 4.Mo(CO) 4 than 4.This is notable as the through-bond coupling paths are shorter in the complex (formally 3 J CP , 2.3 and 2.9 Hz), compared to those in the free ligand 4 ( 5TS J CP , 16.2 and 34.1 Hz).
Crystals of 5 were grown from ethanol.The structure of 5 displays a moderately strained geometry, with a P9•••Sb1 distance of 3.172(3) Å (129% of ∑r covalent , 76% of ∑r vdW ) [24,25] and a splay angle of 15.1(12) • .These parameters indicate that, while the two functional groups in the peri-positions are forced into close proximity, the P•••Sb interaction is primarily repulsive.However, a more detailed look at the peri-region geometry indicates the presence of a weak intramolecular pnictogen bond (n(P)→σ*(Sb-C iPr )), which manifests through a quasi-linear arrangement of the P9•••Sb1−C19 motif (168.5 • , see Figure 3).
Crystals of 8 were obtained by prolonged standing of the crude oily product.The molecule of 8 in the structure displays a similar geometry to 5, with a slightly larger P•••Sb distance of 3.218(2) Å.In contrast to 5, the "homoleptic" substitution pattern of the Sb atom in 8 allows direct comparison of the Sb-C bond lengths for the two n-butyl groups.This reveals that the Sb1−C13 bond length is significantly elongated compared to the Sb1−C17 bond length (2.197(6) vs. 2.100(7) Å).This indicates the donation of electron density (n(P9)) into the (antibonding) σ*(Sb1−C13) orbital, consistent with the formation of a (quasi-linear) pnictogen bond n(P9)→σ*(Sb1-C3), P9  4 were grown from hexane.The molybdenum adopts a (distorted) octahedral geometry as expected, with ligand 4 attached in cis fashion.As above, the P-Mo distance is as expected; however, the Sb-Mo bond length of 2.7007(6) Å, is one of the shortest Sb-Mo bonds known.Of the 518 independent Sb-Mo bonds (in 97 compounds) recorded in the Cambridge Structural Database, only 8 are shorter than the bond in compound 4.Mo(CO) 4 .The three compounds showing the shortest distances (the shortest being 2.64386(19) Å) are all stibine (or halostibine) complexes, possessing a tridentate scaffold combining stibine and phosphine functionalities, with similar constraints as those seen in 4 [29].
A Quantum Theory of Atoms in Molecules (QTAIM) [30,31] analysis was applied to 5 and 8. Bond critical points (BCPs) were located between the Sb1 and P9 atoms for both 5 and 8, indicative of a bonding interaction.Selected QTAIM parameters evaluated at BCPs for these molecules are summarized in Table 2.The Sb1•••P9 BCPs all display a relatively low electron density (ρ BCP ) and a small and positive Laplacian (∇ 2 ρ BCP ), which are typical of interactions between heavier elements [32,33].The bond degree parameter [32] (BD = H BCP /ρ BCP ; H BCP = energy density at BCP) [32,34] and the ratio of |V BCP |/G BCP [32] (V BCP = electronic potential energy at the BCP and G BCP = electronic kinetic energy at the BCP) are two valuable metrics in QTAIM for analysing bonds between heavier elements.The BD indicates the amount of covalency in a bond, with larger negative values denoting a greater covalent interaction [32,34].The P9•••Sb1 interactions in 5 and 8 both show small, negative values, suggesting a weakly covalent interaction (Table 2).|BD| is smaller for 8 than 5, suggesting less covalency in the P9•••Sb1 interaction for 8.This can be rationalised by the Sb(nBu) 2 moiety being more electron-rich than Sb(Ph)iPr, and thus a poorer electron acceptor.The interaction energy (E i = 1 2 V BCP ) [35], which can be used as a rough estimate of bond strength, similarly indicates a weaker P9•••Sb1 interaction in 8.
The |V BCP |/G BCP ratio differentiates between different bond types: purely closedshell interactions such as van der Waals or ionic bonds exhibit |V BCP |/G BCP < 1, while fully covalent interactions show |V BCP |/G BCP > 2. Bonds with intermediate ratios (1 < |V BCP |/G BCP < 2) are termed transit closed-shell interactions, such bonds possess partial covalent character [32].Both 5 and 8 display 1 < |V BCP |/G BCP < 2, with a larger value for 5 than 8.This once again suggests a more covalent P9•••Sb1 in 5 than 8. Crucially, the BD and |V BCP |/G BCP suggest that the P9•••Sb1 interaction in both 5 and 8 is not purely closed shell (i.e., Van der Waals), and that there is some degree of electron sharing between the P and Sb atoms, consistent with a nP→σ*(Sb-C) interaction.Also of note is the difference in QTAIM parameters for Sb1−C13 and Sb1−C17 in 8. Sb−C13 shows a slightly reduced BD, |V BCP |/G BCP and E i compared with Sb−C17 (Table 2), consistent with a weakening of the Sb1-C13 bond due to donation into the Sb-C σ* orbital.This P9•••Sb1 interaction was further probed by an Energy Decomposition Analysis employing Naturalised Orbitals for Chemical Valence (EDA-NOCV) [36][37][38][39].This allows the donor-acceptor interaction between P9 and Sb1 to be visualised and also allows for quantification of the interaction energy.For this analysis, the molecules were divided into two closed-shell fragments: an Acenap(PiPr 2 ) − anion and an SbR 2 + cation.The total interaction energy between these fragments (∆E int ) was computed and divided into terms for ∆E steric , ∆E orb and ∆E disp (Table 3).∆E orb and ∆E disp are the orbital and dispersion interaction energies, respectively.∆E steric is the combined electrostatic attraction and Paulirepulsion energy terms [40].In both 5 and 8, ∆E steric is negative, indicating a significant electrostatic attraction.This is a result of formally assigning the fragments as cationic and anionic.8 is observed to have a slightly smaller ∆E int , ∆E steric , ∆E orb and ∆E disp than 5, which can again be contributed to more electron rich groups on Sb weakening the donor-acceptor interaction.The ∆E orb term can be broken down into pairs of natural orbitals for chemical valence (NOCVs), which represent the orbital interactions between the Acenap(PiPr 2 ) − and SbR 2 + fragments.For each pair of NOCVs, a deformation density plot (∆ρ k ), which represents the flow of electrons between the molecular fragments, and its corresponding energy contribution to ∆E orb , can be determined [36].The first deformation density plots (∆ρ 1 ) for 5 and 8, which have the largest energetic contribution to ∆E orb , are dominated by electron flow from the (anionic) carbon of Acenap(PiPr 2 ) − to the Sb atom.However, ∆ρ 2 and ∆ρ 3 for 5 and 8 both appear to show electron donation from the P lone pair to a Sb−C σ* orbital (Figure 4).The energy contributions of these interactions are ∆ρ 2 = −15.8kcal mol −1 , ∆ρ 3 = −12.1 kcal mol −1 for 5; ∆ρ 2 = −14.4kcal mol −1 , ∆ρ 3 = −11.0kcal mol −1 for 8.These values are likely a significant overestimate of the nP→σ*(Sb-C) interaction energy, as the deformation density plots also show significant contributions from the π-systems of 5 and 8.However, these plots do strongly support the existence of donor-acceptor interactions between P9 and Sb1 in both compounds.Note that blue isosurface (an indicator of accepting electron density) is primarily observed on the Sb-C bond opposite the P-atom and not the other Sb-Ph (5) or Sb-nBu (8) bond (Figure 4).

General Considerations
Unless otherwise stated, all experimental procedures were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques or under an argon atmosphere in a Saffron glove box.Dry solvents were used unless otherwise stated and were either collected from an MBraun SPS-800 Solvent Purification System, or dried and stored according to literature procedures [41].The peri-substituted acenapthene precursors 1

General Considerations
Unless otherwise stated, all experimental procedures were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques or under an argon atmosphere in a Saffron glove box.Dry solvents were used unless otherwise stated and were either collected from an MBraun SPS-800 Solvent Purification System, or dried and stored according to literature procedures [41].The peri-substituted acenapthene precursors 1 [42], 2, 3 and 6 [18] were synthesised according to literature procedures."In vacuo" refers to a pressure of ca. 2 × 10 −2 mbar.

NMR Spectroscopy
All novel compounds were characterised where possible by 1 H, 13 C DEPTQ and 31 P{ 1 H} NMR spectroscopy, including measurements of 1 H{ 31 P}, H-H DQF COSY, H-C HSQC, H-C HMBC and H-P HMBC. 13C{ 1 H} NMR spectra were recorded using the DEPTQ-135 pulse sequence with broadband proton decoupling.Measurements were performed at 20 • C using a Bruker Avance 300, Bruker Avance II 400 or Bruker Avance III 500 (MHz) spectrometer.For both 1 H and 13 C NMR, chemical shifts are relative to Me 4 Si, which was used as an external standard.The residual solvent peaks were used for calibration (CHCl 3 , δ H 7.26, δ C 77.16 ppm; CD 2 Cl 2 , δ H 5.32, δ C 53.84 ppm).For 31 P NMR, 85% H 3 PO 4 in D 2 O (δ P 0 ppm) was used as an external standard. 195Pt NMR was acquired for 2.PtCl 2 , and 1.2 M Na 2 [PtCl 6 ] in D 2 O (δ Pt 0 ppm) was used as the external standard.The NMR numbering scheme is shown in Figure 5.

Other Analyses
Elemental analyses (C, H and N) were performed at London Metropolitan University.High resolution mass spectrometry was performed by the EPSRC UK National Mass Spectrometry Facility (NMSF) at Swansea University using either a Waters Xevo G2-S (ASAP) or a Thermofisher LTQ Orbitrap XL (APCI) mass spectrometer.Electrospray ionisation (ES) spectra were acquired at the University of St Andrews Mass Spectrometry Facility using a Thermo Exactive Orbitrap Mass Spectrometer.Both IR and Raman spectra were collected on a Perkin Elmer 2000 NIR FT spectrometer.KBr tablets were used in IR measurements; powders in sealed glass capillaries were used for Raman spectra acquisitions.Melting (or decomposition) points were determined by heating solid samples in glass capillaries using a Stuart SMP30 melting point apparatus.

Other Analyses
Elemental analyses (C, H and N) were performed at London Metropolitan University.High resolution mass spectrometry was performed by the EPSRC UK National Mass Spectrometry Facility (NMSF) at Swansea University using either a Waters Xevo G2-S (ASAP) or a Thermofisher LTQ Orbitrap XL (APCI) mass spectrometer.Electrospray ionisation (ES) spectra were acquired at the University of St Andrews Mass Spectrometry Facility using a Thermo Exactive Orbitrap Mass Spectrometer.Both IR and Raman spectra were collected on a Perkin Elmer 2000 NIR FT spectrometer.KBr tablets were used in IR measurements; powders in sealed glass capillaries were used for Raman spectra acquisitions.Melting (or decomposition) points were determined by heating solid samples in glass capillaries using a Stuart SMP30 melting point apparatus.

X-ray Diffraction
X-ray diffraction data for compound 2.PtCl2 were collected at 125 K using the St Andrews Automated Robotic Diffractometer (STANDARD) [43], consisting of a Rigaku sealed-tube X-ray generator equipped with a SHINE monochromator [Mo Kα radiation (λ = 0.71075 Å)], and a Saturn 724 CCD area detector, coupled with a Microglide goniometer head and an ACTOR SM robotic sample changer.Diffraction data for compounds 4.Mo(CO) 4 , 5 and 8 were collected at 173 K using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics [Mo Kα radiation (λ = 0.71075 Å)] with an Xta-LAB P200 diffractometer.Intensity data for all compounds were collected using ω steps, accumulating area detector images spanning at least a hemisphere of reciprocal space.Data for all compounds analysed were collected using CrystalClear [44] and processed (including correction for Lorentz, polarization and absorption) using either CrystalClear or CrysAlisPro [45].Structures were solved by direct (SHELXS [46]), Pattterson (PATTY [47]) or charge-flipping (Superflip [48]) methods and refined by full-matrix least-squares against F 2 (SHELXL-2019/3 [49]).Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model.In 5, both isopropyl groups bound to phosphorus were disordered over two positions.Atoms were split and refined with partial occupancies, and restraints to bond distances were required.Crystals of 8 were affected by pseudo-merohedral twinning, showing a twin law of [−0.9999 0.0198 0.0009 −0.0032 0.9992 −0.0203 −0.0007 −0.1315 −0.9981] and a refined twin fraction of 0.489.All calculations were performed using the CrystalStructure interface [50].Selected crystallographic data are presented in Table 4. Geometry optimisations were performed for models 5 and 8 using coordinates derived from their X-ray crystal structures.These models were geometry optimised without restraints using the ORCA 5.0.4 software package [51] utilising the PBE0 density functional [52] and all-electron ZORA corrected [53] def2-TZVP basis sets [54][55][56][57] for all atoms (except Sb), SARC-ZORA-TZVP [58] basis sets for the Sb atoms, along with SARC/J auxiliary basis sets decontracted def2/J up to Kr [59] and SARC auxiliary basis sets beyond Kr.[58,[60][61][62].Gradient corrections were performed with Grimme's 3rd generation dispersion correction [63,64].TightSCF and TightOpt convergence criteria were employed, and the location of true minima in these optimisations was confirmed by frequency analysis, which demonstrated that no imaginary vibrations were present.

Conclusions
The synthetic utility of peri-substituted phosphine-chlorostibines 3 and 6 in reactions with carbon nucleophiles has been demonstrated.Reactions with Grignard reagents or nBuLi proceeded rather cleanly and gave alkyl/aryl and alkyl tertiary stibines 4, 5, 7 and 8 with very good yields.
The coordination chemistry of the selected tertiary phosphine-stibines has also been probed.Two complexes, 2.PtCl 2 and 4.Mo(CO) 4 , have been synthesised.Single-crystal X-ray diffraction confirmed that both the phosphine and the stibine groups are attached to the platinum(II) and Mo(0) centres.
In the phosphine-phosphine peri-substituted species, such as iPr 2 P-Ace-PPh 2 , large magnitude 4TS J PP (180 Hz for the above species) were observed due to the forced overlap of the two lone pairs on the phosphorus atoms [68].As antimony has no spin ½ isotopes, the direct observation of P−Sb couplings (formally 4TS J PSb ) was not possible in the phosphinestibines reported here.However, long-range 5TS J CP couplings of up to 36.8 Hz were observed for carbon atoms attached to Sb atoms in all the phosphine stibines.This indicates the presence of a strong through-space coupling pathway through the phosphorus and antimony atoms (both possessing a lone pair), and the through-bond pathway contribution ( 5 J) is expected to be negligible [69].
The QTAIM analysis supports the existence of a P•••Sb interaction in 5 and 8, which is not purely closed-shell (i.e., Van der Waals), and the visualisation of the deformation densities in an EDA-NOCV analysis supports the view that electron density from the P atom flows towards an apparent Sb-C σ*orbital.

Figure 1 .
Figure 1.Literature Group 15 peri-substituted species mentioned in the introduction.

Molecules 2024 , 20 Figure 3 .
Figure 3. Molecular structures of 5 and 8 with molecules aligned approximately along the acenaphthene plane to show the quasi-linear P•••Sb-C motifs.Hydrogen atoms and minor components of disorder are omitted for clarity.

Crystals of 8
were obtained by prolonged standing of the crude oily product.The molecule of 8 in the structure displays a similar geometry to 5, with a slightly larger P•••Sb distance of 3.218(2) Å.In contrast to 5, the "homoleptic" substitution pattern of the Sb atom in 8 allows direct comparison of the Sb-C bond lengths for the two n-butyl groups.This reveals that the Sb1−C13 bond length is significantly elongated compared to the Sb1−C17 bond length (2.197(6) vs. 2.100(7) Å).This indicates the donation of electron density (n(P9)) into the (antibonding) σ*(Sb1−C13) orbital, consistent with the formation of a (quasi-linear) pnictogen bond n(P9)→σ*(Sb1-C3), P9•••Sb1−C13 angle 167.1°, see Figure 3. Crystals of 2.PtCl2 were grown from dichloromethane/hexane with a solvated molecule of dichloromethane.The platinum atom adopts a distorted square planar geometry, with the P and Sb atoms of ligand 2 bound in a cis fashion.Coordination of the PtCl2 fragment results in elongation of the P•••Sb distance to 3.357(

Figure 3 .
Figure 3. Molecular structures of 5 and 8 with molecules aligned approximately along the acenaphthene plane to show the quasi-linear P•••Sb-C motifs.Hydrogen atoms and minor components of disorder are omitted for clarity.

Figure 4 .
Figure 4. Deformation densities for 5 and 8 associated with nP→σ*(Sb-C) interaction.Charge flow is from the negative isosurface (red) to the positive isosurface (blue).All isosurfaces are plotted at 0.001 au.

Figure 4 .
Figure 4. Deformation densities for 5 and 8 associated with nP→σ*(Sb-C) interaction.Charge flow is from the negative isosurface (red) to the positive isosurface (blue).All isosurfaces are plotted at 0.001 au.

Author
Contributions: L.J.T., E.E.L. and B.A.C. carried out the required synthetic steps, collected all data (except X-ray), and analysed the data.A.M.Z.S., D.B.C. and K.S.A.A. collected the X-ray data and solved the structures.L.J.T. performed all computational analysis.P.K. and B.A.C. designed the study.P.K., L.J.T. and D.B.C. wrote the manuscript.P.K. provided the supervision.All authors have contributed to the proof-reading and editing of the manuscript.All authors have read and agreed to the published version of the manuscript.

Table 1 .
Selected bond distances, displacements, angles and torsion angles for the phosphine-stibines and their metal complexes.

Table 2 .
Selected properties of the electron density at bond critical points according to QTAIM analysis.ρ(r) and ∇ 2 ρ(r) are given in standard atomic units.