Arsolyl-supported intermetallic dative bonding

The first examples of late transition metal η5-arsolyls (L = CO, P(OMe)3; R = Ph, Me, Et, SiMe3; R′ = Ph, H, Me, Et, Me) serve as ditopic donors to extraneous metal centres (M = PtII, AuI, HgII) through both conventional As → M and polar-covalent (dative) Co → M interactions.


Introduction
Transition metal complexes in which an arsolyl ligand 'AsC 4 R 4 ' acts as a pentahapto cyclopentadienyl mimic are limited to [M(CO) 3 (h 5 -AsC 4 Ph 4 )] (M ¼ Mn, Re) and a small number of structurally characterised ferrocene analogues [Fe(h 5 -AsC 4 Me n -H 4Àn ) 2 ] (n ¼ 0, 2, 4), 1 all of which involve d 6 -pseudo-octahedral metal centres. This is in contrast to the chemistry of h 5 -phospholyls which is richly diverse and well-charted across the entire periodic system. 2 Of late, the question of intermetallic polarcovalent (dative) bonding has received considerable attention, 3 but nds its origin in the early observation that d 8 -[Co(CO) 2 (h-C 5 H 5 )] forms a Lewis base/acid adduct with HgCl 2 . 4 While championing the concept of "Metal Bases par excellence" 5 Werner placed particular emphasis on group 9 complexes of the form ], recalling the archetypal Lewis basicity of [Co(CO) 2 (h-C 5 H 5 )]. Given that h 5 -phospholyls may on occasion display P-centred nucleophilicity, 6 and that Mathey has established the viability of late transition metal h 5 -phospholyls, e.g., [Co(CO) 2 (h 5 -PC 4 Ph 2 H 2 )], 7 we have considered whether currently unknown arsolyl complexes of late transition metals with higher d-occupancies might also be viable. Specically, we were intrigued to explore whether these might also serve as Lewis bases towards other metal centres and to what extent the arsenic donor, being typically less nucleophilic than in phosphorus congeners, might augment, support, or competitively compromise resulting metal-metal bonding. Accordingly, we report herein the isolation of the rst late transition metal h 5arsolyl complexes and demonstrate their proclivity towards bridge-assisted metal-metal bond formation.
Heating [Co 2 (CO) 8 ] and a selection of As-phenyl arsoles (1ae, Scheme 1) in reuent THF or n-hexane provides the highly airsensitive arsolyl complexes [Co(CO) 2 (h 5 -AsC 4 R 4 )] (2a-e) in modest yields following strictly anaerobic chromatography. Efforts to increase the isolated yields of 2a-e with extended reaction times or increased reaction temperatures were unsuccessful, however slightly increased yields were obtained by instead employing the more reactive As-chloro arsoles (see ESI †). The precise fate of the As-substituent during these reactions was not evident from the signicant quantities of intractable materials also produced. The cleavage of an As-Ph bond from an arsole has on one previous occasion been observed in the reaction of [Mn 2 (CO) 10 ] with PhAsC 4 Me 2 H 2 , albeit under rather more forcing conditions (150 C, 4 hours). 1c Sub-optimal yields notwithstanding, the syntheses of 2a-e were highly reproducible, affording complexes 2a and 2b as orange solids or 2c-e as dark orange-red liquids at ambient temperature; the latter group underwent substitution with trimethylphosphite in toluene at 100 C to provide the bright orange crystalline complexes [Co(CO){P(OMe) 3 }(h 5 -AsC 4 R 4 )] (2f-h) in high yield.
Selected spectroscopic data for 2a-e and other germane cobalt(I) dicarbonyl complexes are presented in Table 1 The structural models for 2a, 2b, 2f and 2h all conrm the targeted h 5 -arsolyl coordination. In 2a and 2f the ligands are almost symmetrically disposed with respect to the vertical plane which bisects the h 5 -arsolyl ring, whereas for 2b and 2h these are rotated to a position slightly offset from the arsenic-ring centroid vector (ESI †). Consistent with the difference in the covalent radii of carbon (0.76Å) and arsenic (1.19Å), the latter is in each case very slightly displaced (3)(4)(5) from the mean plane dened by the heterocycle carbon atoms, though less than found in the free arsoles (1a: 10.2 ; 1b: 7.10 ). 10 The geometry of the metal and h 5 -arsolyl rings in 2a, 2b, 2f and 2h are of a distorted pentagonal pyramid. The C a -As-C a angles at the arsenic vertices are signicantly contracted (84-87 ) from that of an idealised pentagon (108 ) while the remaining C-C-C angles are in the range 111-114 , and comparable to those found in [Co(CO) 2 (h 5 -C 5 R 5 )] (R ¼ Me, Ph, CH 2 Ph). 8 The results of computational interrogation of the model The HOMOÀ1 is in all cases substantially derived from the Co-d z 2 orbital and readily corroborates the known nucleophilic behaviour of 2 0 CH . For 2 0 CH this is, however, effectively the only orbital that is geometrically disposed to allow the complex to function as a Lewis base since the HOMO is involved with cyclopentadienyl binding. This is also the case with the HOMO of the pnictogenolyl examples however the orbital substantially protrudes radially from the ring. The HOMOÀ1 involves substantial contribution from the pnictogen orbital such that both the HOMO and HOMOÀ1 (and also HOMOÀ2) contribute to a prominent region of electron density localised over these atoms which is reected in the electrostatic potential map for 2 0 As and Table 1 Selected spectroscopic data for complexes prepared in this work, and some previously reported complexes for comparison Co (  the condensed Fukui functions for both arsenic and cobalt (Fig. 2  inset). Furthermore, on descending group 15, the pnictogen A-p z orbital increasingly contributes and this is accompanied by a monotonic increase in the energy the HOMO, HOMOÀ1 and HOMOÀ2 orbitals which should manifest as an increase in the basicity of not only the metal but also the pnictogen. This is intriguingly counterintuitive in that the basicity, nucleophilicity and strength of pnictogen coordination generally decreases for simple pnictanes AR 3 traversing from P to Sb. 12 Compared to phospholyl and arsolyl complexes, h 5 -stibolyl complexes are rarer still, being limited to three ferrocene analogues, but clearly worthy of further study, not least because of the onset of secondary bonding for the heavier pnictogens. 13 To explore the possibility of metal-metal bond-formation, the representative 2c was chosen, commencing with mercuric chloride by analogy with the prototypical and monomeric adduct [Co(HgCl 2 )(CO) 2 (h-C 5 H 5 )]. 4 The reaction of 2c with HgCl 2 in acetone rapidly results in precipitation of the poorly soluble yellow dimer [2c$HgCl(m-Cl)] 2 (3) in high yield (Scheme 2 and Fig. 3).
The dimeric formulation follows from HR-ESI-MS data, which are devoid of ions due to dissociated 2c, in addition to crystallographic analyses of two isomers that differ in the m:sh 5 -arsolyl rings adopting mutually syn or anti positions with respect to the rhomboidal Hg 2 (m-Cl) 2 core. Thus, yellow needles of anti-3 (major) and orange prisms of syn-3 (minor) slowly crystallise together from solutions of 3 in acetone stored at À30 C.
One half of each of the dimeric structures of both anti-3 and syn-3 in the solid state is crystallographically unique due to the centre of the Hg 2 (m-Cl) 2 unit coinciding with either an inversion centre (anti-3 in P2 1 /n) or twofold rotation axis (syn-3 in C2/c). The coordination polyhedra of the Hg II atoms are strikingly different in each isomer: anti-3 features severely distorted trigonal bipyramidal mercury with the arsenic and cobalt atoms  assuming pseudo-axial and -equatorial positions, respectively (s 5 ¼ 0.89), whereas for syn-3 the more sterically congested mercury geometry more closely approaches a square-based pyramid (s 5 ¼ 0.62) with the non-bridging chlorides occupying the eclipsing apices. The As-Hg bond distances are somewhat shorter by ca. 0.1Å in anti-3 (2.633(6)Å) than syn-3 (2.727(9)Å), while the Co-Hg bond distances of 2.670(9)Å (anti) and 2.620(1)Å (syn) are essentially equivalent within crystallographic precision limits. The latter pair are somewhat longer than the sum of covalent radii for the individual elements (2.58 A) and the separation (2.578(4)Å) observed for [Co(HgCl 2 )(-CO) 2 (h 5 -C 5 H 5 )], 4 by virtue of the increased coordination number at mercury. The slight elongation of the Co / Hg interaction here is almost certainly a geometric compromise to accommodate the Hg II centre within the disparate coordination spheres of the As III and Co I donors, rather than indicating any noteworthy electronic phenomena beyond non-directional spodium bonding. 14 From a valence-bond perspective, 2c may be considered to serve as a neutral, 4-electron bidentate ligand with a somewhat narrow bite-angle (ca. 55-56 ). Solution infrared data for each isomer (aer manual separation of crystals) resulted in spectra identical to that of the bulk sample of 3 obtained above, i.e., it appears that upon dissolution in acetonitrile, syn-3 isomerises to anti-3 and the latter conformation is the natural condition of the adduct. Given both isomers have identical QSAR volumes (536Å 3 ), the polarity of syn-3 (dipole ¼ 23.5 D cf. 0 for anti-3) possibly plays a role in its crystal formation.
Though their Lewis acidity is well-documented, neither [Hg(CF 3 ) 2 ] nor [Hg(C 6 F 5 ) 2 ] provided any evidence of detectable adduct formation with 2c. Coordination of 2c to divalent platinum(II) could however be demonstrated in its reaction with cis-[Pt(C 6 F 5 ) 2 (hex)] (hex ¼ h 2 :h 2 -1,5-hexadiene) in CH 2 Cl 2 to provide aer anaerobic chromatography a single isolable orange compound in modest yield. The complex with three n CO absorptions at 2084, 2053 and 2014 cm À1 and the observation of six resonances in the 19 F NMR (ESI †) conrmed the presence of two chemically inequivalent C 6 F 5 groups inconsistent with a simple '2c$Pt(C 6 F 5 ) 2 ' adduct. The X-ray diffraction analysis (Fig. 4) conrmed it to be [2c$cis-Pt(CO)(C 6 F 5 ) 2 ] (4) arising from CO sequestration.
Computational interrogation (uB97X-X/6-31G*/LANL2Dz) of the model complexes [CoPt(m-AsC 4 H 4 )(CO) 3 (CF 3 ) 2 ] (4 0 ) and [Co 2 Au(m-AsC 4 H 4 ) 2 (CO) 4 ] + (5 0 ) (detailed in the ESI †) returns core geometries close to those of 4 and the cation of 5. This analysis reveals molecular orbitals of interest (see ESI Fig. S12-S14 †) that account for the geometrical features of note. For 4 0 , the HOMO and HOMOÀ7 comprise signicant overlap of the Co-d z 2 orbital with platinum, supporting a Co / Pt description (Löwdin bond orders As-Pt/Co-Pt ¼ 0.75/0.41). For 5 0 not only does the HOMOÀ16 adhere to the view of dative Co / Au + bonding (Löwdin bond orders As-Au/Co-Au ¼ 0.78/0.56), but an orbital (HOMOÀ20) has a topology suggestive of the arsenolyl serving as a p-acceptor from gold, a feature also present in the MO scheme for 4 0 . Furthermore, the LUMO which has substantial arsenic character would appear to account for the association of the [Au(C 6 F 5 ) 2 ] À anion at this point which underpins the extended polymeric assembly, though it is unlikely that this persists in solution.

Conclusions
The rst (eight) examples of late transition metal h 5 -arsolyl complexes have been obtained with one example being then employed to explore the possibility of both the metal and arsenic serving as donors to Pt II , Au I and Hg II centres. Although the individual interactions might appear weak, when both act in concert novel bridge-assisted heterometallic assemblies arise with intriguing features.

Data availability
Crystallographic data for structurally characterised compounds have been deposited at the Cambridge Crystallographic Data Centre under CCDC 2130783-2130790 and can be obtained from https://www.ccdc.cam.ac.uk. Spectroscopic data for all new compounds are provided in the ESI accompanying this paper (https://doi.org/10.1039/d2sc01200f).

Author contributions
RMK was responsible for the conceptualisation and execution of the experimental research, the acquisition and critical analysis of the characterisational data and compilation of the original dra. AFH was responsible for funding acquisition, project administration, validation and renements to the manuscript.

Conflicts of interest
There are no conicts to declare.