Rhodium(III) Complexes Featuring Coordinated CF3 Appendages

Abstract 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 J RhF and 2 J PF coupling.

The coordination chemistry of the transition elements is extensive, but notable for the paucity of well-defined complexes featuring explicit CÀF!Mb ondingi nteractions. [1,2] Indeed, the poor ligating characteristics of organofluorine groups, augmented by the inertnesso ft he associated CÀFb onds, lend them to notable application as constituents of weakly coordinating anions and solvents. [3] Of the limited number of structurally characterisede xamples,t he overwhelming majority are based on the electrophilice arly transition metals:w ith A-D particularly notable ( Figure 1). [4,5] Complexes of the platinum group metals are scarce and only E-G feature MÀFc ontacts < 2.5 . [6,7] Buildingo no ur recentw ork, employing the high trans-influence 2,2'-biphenyl (biph) ancillary ligand for the systematics tudy of agostici nteractions, [8] we herein report the synthesis and characterisation of an unprecedented homologous series of late transition metal complexes featuring distinct CF 3 !Mb onding interactions.
To temper the extremelyl ow nucleophilicity of the CF 3 group, we focused our efforts on probing the intramolecular coordination chemistry of this commonly used appendage and identified PPh 2 Ar F as ap rospective ditopic ligand ( Figure 1). [9] MonomericR h III complex[ Rh(biph)(dtbpm)Cl] (dtbpm = bis(ditert-butylphosphino)methane) is an established source of the {Rh(biph)Cl} fragmenti ns olution [8,10] andr eaction with excess PPh 2 Ar F in CH 2 Cl 2 at RT proceeded, as anticipated, with substitutiono ft he 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,i nt he solid state by single-crystal X-ray diffraction,a nd by combustion analysis. Subsequent substitution reactions enabled synthesis of considerablym ore soluble mononucleard erivatives 2-5, which were all isolated in high purity and extensively characterised( Figure 2).
The solid-state structures of 1-4 are all notable for the adoption of distinct CF 3 !Rh bonding interactions, characterisedb y RhÀFc ontacts of 2.36-2.45 ,i ncreasing in the order 2 < 4 < 3 < 1,a nd significant elongation of the bound CÀFb ond (ca. 0.04 ). Therea re very few crystallographicallyc haracterised transition-metal precedents forc oordinationo ft he CF 3 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, featuresashorter contact[ V ÀF = 2.306 (2) ]. [5,11] Coordination of cyclopentadienyl in 5 leads to the nominal monodentate coordination of PPh 2 Ar F ,w ith the CF 3 group projected away from the metalc entre [ffRh-P-C-CCF 3 = 167.7(1)8 and Rh···F > 5 ]d emonstratingt hat this phosphine ligand is sufficiently conformationally flexible as to not enforcet he chelation observed in 1-4.   In CD 2 Cl 2 solution at 298 K, coordinationo fP Ph 2 Ar F in 1-5 was confirmed by 31 PNMR spectroscopy with the associated resonances exhibiting large 103 Rh coupling( 1 J RhP = 124-170 Hz). Further coupling to magnetically equivalent 19 Fn uclei ( 2 J PF % 5Hz) is evident from the 31 P{ 1 H} NMR spectra of 1-4,b ut absent in that of 5,c onsistentw itht he presence of weak and time-averaged CF 3 !Rh interactions in solution. At ambient temperature, fast rotation of the CF 3 groupso nt he NMR time scale and coupling to both 31 Pa nd 103 Rh, with 1 J RhF % 2 J PF are also apparent from the 19 F{ 1 H} NMR spectra of 1-4 (d CF3 À62.8 to À67.6 ppm;3 76 MHz). [12] The transientn ature of the CF 3 ! Rh interaction in solution inferredf rom thesed ata is fully in line with expectation and further vindicated through pronounceds tructural dynamics of asymmetric 1-3 evident by 1 HNMR spectroscopy at 298 K( 400 MHz), that results in higher than expectedt ime-averaged symmetry of the biph ancillary ligand and invokes dissociation of the CF 3 group.E quivalent exchange processesa re presumably occurring in 4,a lthough the spectroscopic signatures are asymptomatic due to the inherently higher symmetry of this complex.
Furtheri nterrogationo f2-5 in CD 2 Cl 2 was possible by variable-temperature NMR spectroscopy (see Figure 3a nd Supporting Information), with progressive cooling from 298 to 185 Kf reezing out the structural dynamics observed for 2 and 3 ( 1 HNMR, 400 MHz), and inducing the onset of decoalescence of the CF 3 resonances ( 19 FNMR, 376 MHz). Although af ull line shape analysis of the latter was not possible, as the slow exchange regime wasn ot reached, the enthalpies of activation for hindered CÀCF 3 bond rotationc ouldb ee stimated from the temperature dependence of the line width (Figures 2a nd   3). [14,15] The activation barriers increasei nt he order 3 < 4 < 2, correlating with the bond lengths observed in the solid state, and are all larger than that measured for 5.O nly minor broadening of the 1 HA r F signals of 2-5 was observed on cooling, ruling out PÀAr F bond rotation on the NMR time scale.
Through the isolation and structural characterisation of Rh III complexes of PPh 2 Ar F 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 CF 3 functional group. Synthesis of thesec omplexes advances the coordination chemistry of weakly interacting organofluorine compounds, and highlightst he use of CÀF!Mb onding interac- Figure 2. Synthesis,structures anddynamic properties of rhodium(III) complexes of PPh 2 Ar F ;[B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 ] À counter anions omittedf or clarity.All reactions were carriedo ut in CH 2 Cl 2 at RT; 1 was isolated in 82 %y ield,and all subsequent substitution reactionsproceededq uantitatively by NMR spectroscopy. Solid-state structuresd rawnw ith thermal ellipsoidsat5 0%,a nd minor disordered components (1 Ph groupi n1 and 4)and Ha tomsa re omitted; symmetry equivalent atoms in 1 are generated by using the operation (4/3Àx,5 /3Ày,2 /3Àz), only one of the two unique but structurally similar cations shownf or 2 and 3 (Z' = 2). [13] Figure 3. Variable-temperature 19 F{ 1 H} NMR spectra of 2 (CD 2 Cl 2 ,3 76 MHz, 298-185K). tions for the stabilisation of transition metal complexes with a low-coordination number.O nt he basis of computational predictions, [16] adducts of this nature have been predicted to be intermediates in the oxidative addition of C(sp 3 )ÀFb onds and our future work will be focused on testing this hypothesis experimentally.