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Scheme 3.3 Synthesis of (8-phenylsulfanylnaphth-1-yl)diphenylphosphine oxide 21, (8- phenylsulfanylnaphth-1-yl)diphenylphosphine sulphide 22, (8-phenylsulfanylnaphth-1-

yl)diphenylphosphine selenide 23.

In a series of parallel reactions to those observed for compound 3, 19 also reacted to form a series of three phosphorus(V) chalcogenides. Under atmospheric conditions the three-coordinate phosphorus atom of 19 oxidised to form the novel compound (8-phenylsulfanylnaphth-1- yl)diphenylphosphine oxide21 (Scheme 3.3).

Fig. 3.4 The crystal structure of (8-phenylsulfanylnaphth-1-yl)diphenylphosphine oxide 21.

Reaction of 19 with one equivalent of sulfur in toluene produced the novel (8- phenylsulfanylnaphth-1-yl)diphenylphosphine sulfide 22. Similarly 19 reacted under reflux with one equivalent of selenium in toluene to produce the novel (8-phenylsulfanylnaphth-1- yl)diphenylphosphine selenide 23 (Scheme 3.3).5 _{For all three compounds,} 31_{P NMR showed}

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The 31_{P NMR signals for the three compounds were found to lie at similar values to the S(ethyl)}

chalcogenides with all three 4-coordinate phosphorus compounds seeing a downfield shift compared to the 3-coordinate ligand 19 [19 PR3: δ = -5.3 ppm, 21 R3P=O: δ = 37.0 ppm, 22

R3P=S: δ = 52.5 ppm, 23 R3P=Se: δ = 42.4 ppm]. The NMR signals for the three compounds are in

accord with values for similar compounds found in the literature.6 _{The X-ray structures of the three}

compounds were studied (Figure 3.4, 3.6 and 3.8) and compared to the structures of phosphine 19

and the S(ethyl) chalcogenides (16-18). All three compounds show a greater degree of steric strain relief than the non-oxidised phosphine 19 as indicated by the extent of molecular distortion in their structures.

Comparing the peri-distances for Nap[PPh2][SEt] and Nap[PPh2][SPh] and their mono-oxides

3.190(1) 3.191(1) 3.1489(9) 3.228(2) 3.208(1) 3.135(1) 3.0330(7) 2.947(2) 2.974(1) 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 peri-distance [Å] PPh2SR 18 R = Et 23 R = Ph Se

Fig. 3.5 Comparing the peri-distances for the Nap[PPh2][SR] and Nap[E=PPh2][SR] compounds.

Intramolecular peri-distances (Figure 3.5) are greater in all three compounds compared to 19 with oxide 21 and selenide 22 having lengths comparable to their S(ethyl) counterparts. The peri-atoms in selenide 23 lie closer together than expected with the peri-distance similar in size to the sulfide compounds [19 3.0339(13) Å, 21 3.1489(9) Å, 22 3.1909(1) Å, 23 3.190(1) Å]. The length of the phosphorus chalcogen double bonds in the S(phenyl) oxides are in accord with average literature values and very similar to the values found for the S(ethyl) oxides, increasing in length with the size of the chalcogen atom [P=O 1.492(2) Å 21,P=S 1.9585(12) Å 22, P=Se 2.1181(11) Å 23].7

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Fig. 3.6 The crystal structure of (8-phenylsulfanylnaphth-1-yl)diphenylphosphine sulfide 22.

Figure 3.7 shows a comparison of the in-plane distortion occurring in phosphines 3 and 19 and their respective chalcogenides by contrasting the sums of the peri-region angles (Table 3.3). The S(ethyl) chalcogenides have all undergone a greater amount of in-plane distortion compared to the free ligand 3, with sulfide 17 showing a lower than expected value. Conversely the three S(phenyl) chalcogenides all display less distortion than 19 but the degree of distortion increases with increasing size of the chalcogen atom attached to P(1).

Comparison of in-plane distortion in Nap[PPh2][SR] and Nap[E=PPh2][SR] compounds

364 366 368 370 372 374 376 378 380 su m of t he pe ri -r egi on an gl es [ °]

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Fig. 3.8 The crystal structure of (8-phenylsulfanylnaphth-1-yl)diphenylphosphine selenide 23.

Out-of-plane distortion is observed in all three compounds to a similar extent and is considerably greater than in 19 with the peri-atoms lying at similar distances from the naphthalene plane (Figure 3.9). The degree of deviation is also similar to the S(ethyl) chalcogenides. In all three cases the P(1) atom lies above the plane and the S(1) atom lies below the naphthalene plane [P(1)

0.6312(37) Å 21, 0.633(4) Å 22, 0.621(47) Å 23; S(1) -0.582(4) Å 21, -0.451(4) Å 22, -0.433(5) Å

23].

Out-of-plane distortion in the Nap[PPh2][SR] and Nap[E=PPh2][SR] compounds

-1.2 -0.7 -0.2 0.3 0.8 de vi at ion of th e pe ri -at om s ab ove an d be low th e nap ht hal en e pl an e [Å ]

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The chalcogen atoms bonded to P(1) all lie above the plane of the naphthalene with distances from the plane increasing with the increasing size of the atom [O(1) 1.6231(46) Å 21, S(2) 2.1977(43) Å 22, Se(1) 2.3457(51) Å 23]. Following a similar orientation to the S(ethyl) chalcogenides, the chalcogen atom bonded to P(1) aligns above the peri-gap in close proximity to S(1) (Figure 3.10) with the non-bonded S(1)···E distance increasing as the size of the chalcogen atom increases [S(1)···O(1) 2.9612(17) Å 21, S(1)···S(2) 3.3142(11) Å 22, S(1)···Se(1) 3.3974(10) Å 23].

Fig. 3.10 The crystal structures of the (8-phenylsulfanylnaphth-1-yl)diphenylphosphine chalcogenides 21,22 and23 showing the non-bonded S(1)···E distance.

The intramolecular S(1)···E distances are comparable to those found for the S(ethyl) oxides and smaller than the sum of the van der Waals radii for the two interacting atoms [SO = 3.32 Å, SS

3.60 Å, SSe 3.70 Å].8 _{This once again suggests the non-bonded distance is close enough for}

possible weak intramolecular interactions to occur.9

The two torsion angles around the C5-C10 central naphthalene bond in the three compounds shows the naphthalene ring systems of the chalcogenides are considerably more buckled than phosphine 19 with an increase in the planarity moving from oxide 21 to selenide 23 (Figure 3.11). The degree of buckling of the naphthalene ring in 21 is greater than in its S(ethyl) counter part 16. However, sulfide 22 is much more planar than 17. The two selenide compounds (18 and 23) have a similar degree of buckling in the naphthalene ring [21 171.7(2)°, 170.3(2)°, 22 173.7(2)°, 173.4(2)°, 23 175.4(3)°, 173.1(3)°].

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Fig. 3.11 The crystal structures of the (8-phenylsulfanylnaphth-1-yl)diphenylphosphine chalcogenides 21,22 and23 showing the buckling of the naphthalene planes.

Neighbouring aromatic ring systems tend to associate through π-π non-bonded inter- and

intramolecular interactions known as π-π stacking.4 These non-covalent aromatic-aromatic/π-π

interactions are important forces similar to hydrogen bonding. They play a key role in self assembly and molecular recognition processes,10 when extended structures are formed from building blocks with aromatic moieties. Molecular associations utilising π-π stacking have been

well documented in organic,11 _biological12 _{and polymer chemistry.}13

Weaker than the hydrogen bonds, calculations give an energy of about 2 kJ mol-1 _{for a typical}

aromatic-aromatic π-stacking interaction with typical hydrogen bonds between neutral molecules

ranging from 15-40 kJ mol-1_.14 _{Aromatic rings can be arranged and interact either in a stacked}

arrangement or an edge- or point-to-face arrangement, which is a T-shaped conformation. The stacked arrangement known as face-to-face can be arranged with perfect alignment or slipped packing, known as parallel displacement.4 _{The T-shaped conformation is a result of C-H···}

π

interactions.15 _{Typical centroid-centroid}

π-stacking distances range between 3.3-3.8 Å (Figure

3.12).4

Fig. 3.12 Schematic illustration of perfect alignment face-to-face, offset or slipped packing (parallel displacement) and point-to-face (T-shaped) arrangements in π-π stacking.4

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The structures of the chalcogenides show distinct differences in the position and orientation of the S(phenyl) moiety when compared to that of 19 and also with one another. The small amount of out-of-plane distortion in 19 aligns the peri-atoms with the naphthalene plane so the phenyl rings on S(1) and P(1) point in the same direction and align so they are stacked in a close to perfect alignment. The reason no π-π stacking occurs is the large centroid···centroid distance and the

twisting of the phenyl rings and thus no uniform stacked arrangement.

Conversely in the chalcogenides, the repulsive distortion taking place to accommodate the increasingly large chalcogen atoms forces the S(1) and P(1) atoms to lie on opposite sides of the naphthalene plane. Again both phenyl rings point in the same direction. However this time they align in a slipped packing arrangement (Figure 3.13) due to the difference in the position of the two peri-atoms. This is extenuated in oxide 21 due to the degree of buckling in the naphthalene backbone. The phenyl rings in oxide 21 do not align parallel; both rings twist in towards one another and the distance between the two centroids is much higher than for known π-π stacking

(3.3-3.8 Å)4 _{resulting in no observed}

π-π interaction [Cg(17-22)···Cg(23-28):21 4.397(1) Å].

Fig. 3.13 The crystal structures of the (8-phenylsulfanylnaphth-1-yl)diphenylphosphine chalcogenides 21,22 and23 showing orientation of phenyl rings and possible π-π stacking.

The alignment of the rings in 22 and 23 is closer to a parallel alignment and similar to the face-to- face offset arrangement (Figure 3.13).4 _{The distances between the two interacting centroids}

however is slightly longer than the range for typical centroid-centroid π-stacking (3.3-3.8 Å)4 and

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Table 3.3 Bond lengths [Å] and angles [°] for (8-phenylsulfanylnaphth-1-yl)diphenylphosphine chalcogenides21-23. _______________________________________________________________________________ S(1)···P(1) 3.1489(9) S(1)···P(1) 3.191(1) S(1)···P(1) 3.190(1) P(1)-C(1) 1.835(3) P(1)-C(1) 1.837(3) P(1)-C(1) 1.836(4) S(1)-C(9) 1.777(3) S(1)-C(9) 1.779(3) S(1)-C(9) 1.782(4) P(1)=O(1) 1.492(2) P(1)=S(2) 1.9585(12) P(1)=Se(1) 2.1181(11) S(1)···O(1) 2.9612(17) S(1)···S(2) 3.3142(11) S(1)···Se(1) 3.3974(10) P(1)-C(1)-C(10) 124.6(2) P(1)-C(1)-C(10) 124.2(2) P(1)-C(1)-C(10) 125.0(3) C(1)-C(10)-C(9) 126.2(3) C(1)-C(10)-C(9) 127.3(3) C(1)-C(10)-C(9) 126.6(3) S(1)-C(9)-C(10) 121.6(2) S(1)-C(9)-C(10) 122.2(2) S(1)-C(9)-C(10) 122.5(3) Σ = 372.4(5) Σ = 373.7(5) Σ = 374.1(6) C(9)-S(1)-C(23) 102.66(15) C(9)-S(1)-C(23) 101.96(16) C(9)-S(1)-C(23) 101.51(19) O(1)-P(1)-C(1) 114.34(14) S(2)-P(1)-C(1) 113.09(10) Se(1)-P(1)-C(1) 112.29(12)

Distance from naphthalene mean plane [ Å]

P(1) 0.6312(37) P(1) 0.633(4) P(1) 0.621(5) S(1) -0.582(4) S(1) -0.451(4) S(1) -0.433(5) O(1) 1.6231(46) S(2) 2.1977(43) Se(1) 2.3457(51) Torsion angle C:(6)-(5)-(10)-(1) 171.7(2) C:(6)-(5)-(10)-(1) 173.7(2) C:(6)-(5)-(10)-(1) 175.4(3) C:(4)-(5)-(10)-(9) 170.3(2) C:(4)-(5)-(10)-(9) 173.4(2) C:(4)-(5)-(10)-(9) 173.1(3)

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