Through-conjugation of two phosphaalkyne ( ‘ C u P ’ ) moieties mediated by a bimetallic sca ff old †

Through-conjugation of two phosphaalkyne moieties within an isolable molecule is demonstrated for the first time with the synthesis of [{Ru(dppe)2}2{μ-(C[triple bond, length as m-dash]C)2C6H4-p}(C[triple bond, length as m-dash]P)2], via base-induced desilylation of [{Ru(dppe)2}2{μ-(C[triple bond, length as m-dash]C)2C6H4-p}(η1-P[triple bond, length as m-dash]CSiMe3)2]2+. The nature of the cyaphide ligands and their influence upon the bimetallic core are studied electrochemically.

The optimized gas-phase geometries of 2 2+ and 3 (see ESI †) 22 both exhibit slightly greater linearity about the metal centres and bridge when compared with the solid-state structure of 2 2+ , alongside marginally longer CuP linkages (∼1.58 Å).These features are consistent with a prevalence of packing effects in the solid state, as noted previously for several η 1 -PuCR complexes, 20,23 and for our precedent cyaphide-alkynyls. 16The calculated CuP stretching mode for 3 (asym.ν CuP 1224 cm −1 ) also compares well with experiment (ν CuP 1247 cm −1 ).Notably, the experimentally observed frequency reflects a slightly stronger CuP linkage for 3 than in [RuH(dppe) 2 (CuP)] (ν CuP 1239 cm −1 ), 20a attributable to competition with the trans-alkynyl for Ru → π* donation.Indeed, we noted this previously for cyaphide-alkynyls, though to a greater extent (ν CuP 1255, 1260 cm −1 ), 16 suggesting a reduced competition within the bimetallic scaffold.
The frontier orbitals of 2 2+ and 3 (Fig. 2) show similarities, the HOMO in each case being dominated by the bridging π-system (76%, 2 2+ ; 54% 3) with a modest contribution from the metals (14% 2 2+ ; 26% 3).Notably, the HOMO of 3 also includes contributions from π CuP (14%), which engage in outof-phase mixing with the Ru (d xy , d xz ), π CuC and π Ar orbitals, consistent with some level of through-conjugation.The contri-  butions from π CuP increase appreciably in the mutually degenerate HOMO−1 and HOMO−2 (∼25%, see ESI †), lying 0.36 eV below the HOMO, albeit without involvement of the bridging arene (1%).In marked contrast, there is negligible contribution (<10%) from the η 1 -PuCSiMe 3 moieties of 2 2+ to any occupied frontier orbitals, their involvement becoming significant only in the appreciably stabilized HOMO−3 and HOMO−4, lying ca.1.4 eV below the HOMO.Finally, in respect of 3, we note that the terminal cyaphidic lone-pairs manifest in the HOMO−14 and HOMO−15, being stabilised by ca. 2 eV relative to the HOMO.This is entirely consistent with expectation, being similar to our previous observations, 16 and those for phosphaalkynes more generally. 24Additionally, NBO calculations suggest these to reside in orbitals of ca.75% s and 25% p character, as is typical of phosphaalkynes.
As is typical of complexes with the Ru(dppe) 2 scaffold, the latter dominates the virtual orbitals of 3, which are mostly centred on the dppe ligands; the bridge contributes marginally to LUMO+12 and LUMO+14, lying 4 eV above the HOMO.In contrast, while the LUMO/LUMO+1 of 2 2+ are again dominated by the Ru(dppe) 2 framework, LUMO+2 is centred on the unsaturated core, with appreciable contributions from π* CuP (60%) and the bridge (15%).This is reflected in the electronic spectrum of 2 2+ , assigned in comparison with those derived from TD-DFT studies, 25 calculating the first 200 excited states.This offers a fair approximation of the observed UV spectra for 2 2+ and 3 (within limitations of the model), providing sufficient correlation to assist in the assignment of some key features.Thus, a feature at 350 nm (28 571 cm −1 ) includes significant contribution from LLCT bands (π CuC → π* Ar and π CuC → π* CuP ) with marginal involvement of intraligand CT (π CuC → π* CuC ), alongside the dominant MLCT and LLCT associated with excitation from the HOMO/HOMO+1 to low-lying dppe-based orbitals.A second feature around 260 nm (38 462 cm −1 ) is primarily composed of ILCT within the dppe scaffold (<HOMO−10 → LUMO), but with additional contribution from π CuP → π* CuP ILCT and π Ar → π* CuP LLCT (HOMO−3 → LUMO+5).In contrast, features in the UV/Vis spectrum of 3 around 370 nm (27 027 cm −1 ) and 250 nm (40 000 cm −1 ) are wholly dominated by MLCT and LLCT transitions to the dppe scaffold, with marginal contributions from ILCT within the bridging π-framework; contributions from transitions to the high-lying π* CuP (LUMO+36 to LUMO+39) are negligible.
The redox behaviours of 2 2+ and 3 were explored using cyclic voltammetry (Table 2 and ESI †), both compounds exhibiting two distinct oxidative events, which can be assigned (trivially 26 ) to sequential generation of the Ru III /Ru II and Ru III /Ru III species.For 2 2+ an initial quasi-reversible oxidation occurs at significantly more anodic potential than the corresponding (reversible) feature of 1, presumably a corollary of its cationic nature.The second (irreversible) oxidation is similarly shifted to more positive potential, 27 and demonstrates an appreciable stability for the mixed valence state [2 2+ ] + , K c being comparable in magnitude to that of [1] + and related terminal alkynyls.19e,28 In the case of 3, two irreversible oxidations are observed, the initial event showing a slight anodic shift relative to 1, and indeed related alkynyl systems; 19e,28 the second occurs at lower potential than the corresponding oxidation of [1] + .On the reverse scan, an irreversible reduction process is observed at heavily cathodic potential.Notably, the diminished separation of the oxidative events indicates a reduced stability for the mixed valence state ([3] + ) in comparison to [1] + and, indeed, related alkynyl complexes and [2 2+ ] + , K c being two-orders of magnitude lower than for its counterparts.19e,28 Notwithstanding, some stability is apparent, which implies some retention of the electronic coupling characteristic of the "Ru 2 {μ-(CuC) 2 C 6 H 4 -p}" scaffold, albeit diminished by the seemingly electron-acceptor character of the cyaphide ligand.

Conclusions
In conclusion, we have described the first isolable compound to incorporate two 'CuP' moieties as part of the same conju-   (3).The electronic spectrum shows a dominance of LLCT and MLCT transitions from the bridge and phosphacarbon moieties to the dppe scaffold, with negligible ILCT within the π-system.The redox properties of 3 are more interesting and suggest some electron-acceptor character for the cyaphide ligand.While its presence leads to irreversible redox behaviour and serves to destabilize the mixed-valent state [3] + , the retention of electronic coupling within the bimetallic core provides initial conceptual validation for the incorporation of the cyaphide ligand into electro-active complexes.This will require engineering of appropriately stabilizing ancillary scaffolds, a challenge with which we are currently engaged.

Table 1
Comparative experimental and calculated NMR spectroscopic data a Increase in δ P due to SiPh 3 vs.SiMe 3 .d GIAO method with the PBE functional (lanl2dz for Ru; 6-31G** for all other atoms); referenced to H 3 PO 4 or Me 4 Si at the same level of theory.