C , C 0 -Ru to C , B 0 -Ru isomerisation in bis(phosphine)Ru complexes of [1,1 0 -bis( ortho - carborane)] †‡

We report herein the first example of the controlled isomerisation of a C , C 0 -bound (to metal) bis( ortho -carborane) ligand to C , B 0 - bound with no other change in the molecule. Since the C and B vertices of carboranes have diﬀerent electron-donating properties this transformation allows the reactivity of the metal centre to be fine-tuned.

Carboranes are exceedingly versatile ligands to transition-metals. 1 Deboronation of the neutral carborane [closo-1,2-C 2 B 10 H 12 ] to the anion [nido-7,8-C 2 B 9 H 11 ] 2À affords a ligand which is isolobal with the ubiquitous Cp À , able to bind to metals in full Z 5 fashion, 2 or Z o5 in slipped metallacarboranes. 3Alternatively carboranes, particularly anionic carboranes, are able to co-ordinate metals through one, 4 two 5 or three 6 B-H,M B-agostic interactions, taking advantage of the hydridic nature of H atoms bonded to B. Finally direct C-M 7 and B-M 8 sigma bonding is well established and is sometimes accompanied by B-H,M B-agostic bonding from adjacent B atoms. 9 Whether a carborane binds directly to a metal, or to a substituent which is subsequently linked to a metal, through a C or B vertex is particularly important in that, everything else being equal, a B-bound carborane is more electron-donating than a C-bound carborane. 10This affords two isomeric forms of the same ligand which are isosteric but not isoelectronic, and recently this has been exploited to fine-tune the properties of metal-carborane complexes. 11is(carboranes) are molecules composed of two carborane moieties connected by a direct C-C, C-B or B-B bond and, of the various possible bis(carboranes), [1-(1 0 -closo-1 0 ,2 0 -C 2 B 10 H 11 )-closo-1,2-C 2 B 10 H 11 ] or more simply [1,1 0 -bis(ortho-carborane)], is the most studied and has undergone a resurgence of interest in recent years. 12Following double deprotonation at the protonic C2H and C2 0 H sites, [1,1 0 -bis(ortho-carborane)] can be used as a k 2 chelating ligands in both homoleptic 13 and heteroleptic 9b,11c,d,12f,14 transition-metal complexes.
In 2016 we reported catalytically-active (arene)Ru complexes of doubly-deprotonated [1,1 0 -bis(ortho-carborane)] in which the metal coordination was completed by a B3 0 -H,Ru B-agostic interaction.9b Reaction of these compounds with phosphine (2 Â PPh 3 or dppe) resulted in displacement of the arene, coordination of the phosphine and a change in the ligating mode of the bis(carborane) from X 2 (C,C 0 )L (L = agostic interaction) to X 2 (C,B 0 )L, the first time C,B 0 ligation of [1,1 0 -bis(orthocarborane)] had been observed.9b Subsequently, Spokoyny and co-workers reported the synthesis of an isomeric mixture of Pt complexes of [1,1 0 -bis(ortho-carborane)] with bipyridyl coligands; 11c in one isomer the bis(carborane) was C,C 0 -bound and in the other it was C,B 0 -bound (subsequently he was able to prepare exclusively the C,C 0 -bound isomer by using a different synthetic strategy).11d Heating the mixture 'under forcing conditions' did not drive it to one isomer suggesting that the two isomers were formed via different pathways.
Thus, although it is potentially of great interest to be able to isomerise bis(carborane) from C,C 0 -bound to a metal centre to C,B 0 -bound under controlled conditions, no system has so far achieved this.We now describe the controlled isomerisation of a C,C 0 -bound bis(phosphine) ruthenium complex to its C,B 0bound isomer.
The room temperature reaction of [Ru(k 3 -2,2 0 ,3 0 -{1-(1 0closo-1 0 ,2 0 -C 2 B 10 H 10 )-closo-1,2-C 2 B 10 H 10 })(p-cymene)] (I) with 5 equivalents of PMePh 2 in THF produced a deep-red solution from which both red and yellow components were isolated by preparative thin-layer chromatography (TLC).Although the yellow product appeared stable to work-up, repeated chromatography of the red product (at room temperature) always afforded a small amount of the yellow species, implying that the red and yellow species were related as kinetically-and thermodynamicallystable isomers.
Repeating the reaction at 0 1C, eliminating the chromatographic work-up and crystallising the product at À20 1C allowed the red species (1) to be isolated in good yield (80%) in pure form.§ Mass spectrometry of 1 gave a molecular ion consistent with displacement of the p-cymene ligand of I by two PMePh 2 ligands.Although the 11 B{ 1 H} NMR spectrum at À50 1C was largely uninformative the 1 H spectrum revealed, in addition to multiplet resonances associated with the Ph groups, two doublets arising from the two PMePh 2 units, implying the two phosphine ligands are inequivalent and confirmed by the presence of two mutual doublets, J PP = 28.3 Hz, in the 31 P{ 1 H} NMR spectrum.Importantly, the 1 H{ 11 B} NMR spectrum showed, in addition to resonances between 3 and À1 ppm associated with cage BH exo atoms, a doublet resonance at À3.27 ppm integrating for 1H and indicative of B-H,Ru (showing coupling to only one P atom).Notably absent from the 1 H and 1 H{ 11 B} spectra of 1 was a resonance arising from cage CH.
Collectively these data suggest that in 1 the bis(orthocarborane) unit is bound to the Ru atom in X 2 (C,C 0 )L mode, i.e. via both cage C atoms, unlike the situation in the previously isolated PPh 3 and dppe analogues, 9b and this was subsequently confirmed by crystallographic analysis (Fig. 1).¶ The bis(carborane) unit is indeed bonded to the metal atom via s bonds from C2 and C2 0 and a B3 0 -H3 0 ,Ru B-agostic interaction; thus compound The geometry at Ru is approximately square-pyramidal (C2 apex).The Ru-C2 0 s bond is particularly distorted, as evidenced by the angle Ru1-C2 0 -P ca.1341 cf.Ru1-C2-Q ca.1641 (P and Q are the centroids of the primed and unprimed icosahedra, respectively), presumably as a result of the need to accommodate C2 0 and the B3 0 H3 0 unit in two cis ligand positions.The Ru-C bond lengths are significantly different (shorter to C2), as are the Ru-P bond lengths (shorter to P1), in both cases reflecting the relative trans influences of the ligands (or vacant site) opposite.
Solutions of 1 slowly change from deep red to yellow in colour as the compound isomerises to a new species 2, a process easily followed by 31 P{ 1 H} NMR spectroscopy with two new higher-frequency doublets growing in at the expense of the original ones.At room temperature in CD 2 Cl 2 the conversion is typically 15% after 6 h but is accelerated on heating (ca.75% conversion after 2 h at 40 1C) and retarded on addition of excess phosphine (ca.10% conversion after 24 h).
Compound 2 can be conveniently prepared in good yield (64%) by repeating the original synthesis at room temperature and then stirring for ca. 2 h at 40 1C followed by work-up involving column chromatography.8Mass spectrometry is fully consistent with 2 being an isomer of compound 1.The 11 B{ 1 H} NMR spectrum of 2 is again relatively uninformative save that, as for 1, the chemical shifts imply a closo cage.The 1 H NMR spectrum of 2 again reveals two sets of doublets for the methyl protons of the PMePh 2 ligands and, additionally, an integral-1 resonance assigned to C cage H which unfortunately overlaps with the high-frequency component of one of the CH 3 doublets (d 1.85 ppm).However, further evidence for a cage {CH} unit derives from a resonance in the 13 C NMR spectrum at d 67.5 ppm assigned as CH by DEPT spectroscopy.The presence of a B-H,Ru interaction in 2 was established by the observation of an integral-1 doublet at À3.99 ppm in the 1 H{ 11 B} NMR spectrum.
Thus the NMR data imply an X 2 (C,B 0 )L bonding mode for the bis(ortho-carborane) unit in 2 as has previously been established for the PPh 3 and dppe analogues, 9b and this was subsequently confirmed by an X-ray diffraction study (Fig. 2).**Crystals of 1 and 2 (both studied as their 0.5CH 2 Cl 2 solvates) are isomorphous and at a molecular level the two species differ only in the relative positions of a C and B atom in one cage.