Rhodium(III) Complexes Featuring Coordinated CF3 Appendages.

The synthesis and characterisation of a homologous series of rhodium 2,2'-biphenyl complexes featuring intramolecular dative bonding of the nominally inert and weakly coordinating trifluoromethyl group are described. Presence of these interactions is evidenced in the solid state using X-ray diffraction, with Rh-F contacts of 2.36-2.45 Å, and in solution using NMR spectroscopy, through hindered C-CF3 bond rotation and the presence of time-averaged 1 JRhF and 2 JPF coupling.

The coordination chemistry of the transition elements is extensive, but notable for the paucity of well‐defined complexes featuring explicit C−F→M bonding interactions.1, 2 Indeed, the poor ligating characteristics of organofluorine groups, augmented by the inertness of the associated C−F bonds, lend them to notable application as constituents of weakly coordinating anions and solvents.3 Of the limited number of structurally characterised examples, the overwhelming majority are based on the electrophilic early transition metals: with A–D particularly notable (Figure 1).4, 5 Complexes of the platinum group metals are scarce and only E–G feature M−F contacts<2.5 Å.6, 7 Building on our recent work, employing the high trans‐influence 2,2′‐biphenyl (biph) ancillary ligand for the systematic study of agostic interactions,8 we herein report the synthesis and characterisation of an unprecedented homologous series of late transition metal complexes featuring distinct CF3→M bonding interactions.

Figure 1

Selected examples of structurally characterised early and platinum group metal complexes featuring explicit C−F→M bonding interactions.

To temper the extremely low nucleophilicity of the CF3 group, we focused our efforts on probing the intramolecular coordination chemistry of this commonly used appendage and identified PPh2ArF as a prospective ditopic ligand (Figure 1).9 Monomeric RhIII complex [Rh(biph)(dtbpm)Cl] (dtbpm=bis(di‐tert‐butylphosphino)methane) is an established source of the {Rh(biph)Cl} fragment in solution8, 10 and reaction with excess PPh2ArF in CH2Cl2 at RT proceeded, as anticipated, with substitution of the small bite‐angle diphosphine alongside precipitation of chloro‐bridged dinuclear complex 1 (Figure 2). The structure and purity of this sparingly soluble dimer was corroborated in (dilute) solution by NMR spectroscopy, in the solid state by single‐crystal X‐ray diffraction, and by combustion analysis. Subsequent substitution reactions enabled synthesis of considerably more soluble mononuclear derivatives 2–5, which were all isolated in high purity and extensively characterised (Figure 2).

Figure 2

Synthesis, structures and dynamic properties of rhodium(III) complexes of PPh2ArF; [B(3,5‐(CF3)2C6H3)4]− counter anions omitted for clarity. All reactions were carried out in CH2Cl2 at RT; 1 was isolated in 82 % yield, and all subsequent substitution reactions proceeded quantitatively by NMR spectroscopy. Solid‐state structures drawn with thermal ellipsoids at 50 %, and minor disordered components (1×Ph group in 1 and 4) and H atoms are omitted; symmetry equivalent atoms in 1 are generated by using the operation (4/3−x, 5/3−y, 2/3−z), only one of the two unique but structurally similar cations shown for 2 and 3 (Z′=2).13

The solid‐state structures of 1–4 are all notable for the adoption of distinct CF3→Rh bonding interactions, characterised by Rh−F contacts of 2.36–2.45 Å, increasing in the order 2<4<3<1, and significant elongation of the bound C−F bond (ca. 0.04 Å). There are very few crystallographically characterised transition‐metal precedents for coordination of the CF3 appendage and, to the best of our knowledge,1, 2 only first‐row adduct D (Figure 1), bearing two rigid 2,4,6‐tris(trifluoromethyl)‐phenyl ligands, features a shorter contact [V−F=2.306(2) Å].5, 11 Coordination of cyclopentadienyl in 5 leads to the nominal monodentate coordination of PPh2ArF, with the CF3 group projected away from the metal centre [∠Rh‐P‐C‐CCF3=167.7(1)° and Rh⋅⋅⋅F>5 Å] demonstrating that this phosphine ligand is sufficiently conformationally flexible as to not enforce the chelation observed in 1–4.

In CD2Cl2 solution at 298 K, coordination of PPh2ArF in 1–5 was confirmed by 31P NMR spectroscopy with the associated resonances exhibiting large 103Rh coupling (1 J RhP=124–170 Hz). Further coupling to magnetically equivalent 19F nuclei (2 J PF≈5 Hz) is evident from the 31P{1H} NMR spectra of 1–4, but absent in that of 5, consistent with the presence of weak and time‐averaged CF3→Rh interactions in solution. At ambient temperature, fast rotation of the CF3 groups on the NMR time scale and coupling to both 31P and 103Rh, with 1 J RhF≈2 J PF are also apparent from the 19F{1H} NMR spectra of 1–4 (δ CF3 −62.8 to −67.6 ppm; 376 MHz).12 The transient nature of the CF3→Rh interaction in solution inferred from these data is fully in line with expectation and further vindicated through pronounced structural dynamics of asymmetric 1–3 evident by 1H NMR spectroscopy at 298 K (400 MHz), that results in higher than expected time‐averaged symmetry of the biph ancillary ligand and invokes dissociation of the CF3 group. Equivalent exchange processes are presumably occurring in 4, although the spectroscopic signatures are asymptomatic due to the inherently higher symmetry of this complex.

Further interrogation of 2–5 in CD2Cl2 was possible by variable‐temperature NMR spectroscopy (see Figure 3 and Supporting Information), with progressive cooling from 298 to 185 K freezing out the structural dynamics observed for 2 and 3 (1H NMR, 400 MHz), and inducing the onset of decoalescence of the CF3 resonances (19F NMR, 376 MHz). Although a full line shape analysis of the latter was not possible, as the slow exchange regime was not reached, the enthalpies of activation for hindered C−CF3 bond rotation could be estimated from the temperature dependence of the line width (Figures 2 and 3).14, 15 The activation barriers increase in the order 3<4<2, correlating with the bond lengths observed in the solid state, and are all larger than that measured for 5. Only minor broadening of the 1H ArF signals of 2–5 was observed on cooling, ruling out P−ArF bond rotation on the NMR time scale.

Figure 3

Variable‐temperature 19F{1H} NMR spectra of 2 (CD2Cl2, 376 MHz, 298–185 K).

Through the isolation and structural characterisation of RhIII complexes of PPh2ArF 1–4 we have demonstrated the ability of the late‐transition‐metal complexes to form well‐defined, albeit weakly bound, adducts of the widely employed CF3 functional group. Synthesis of these complexes advances the coordination chemistry of weakly interacting organofluorine compounds, and highlights the use of C−F→M bonding interactions for the stabilisation of transition metal complexes with a low‐coordination number. On the basis of computational predictions,16 adducts of this nature have been predicted to be intermediates in the oxidative addition of C(sp3)−F bonds and our future work will be focused on testing this hypothesis experimentally.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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