Fluoroarene Complexes with Small Bite Angle Bisphosphines: Routes to Amine–Borane and Aminoborylene Complexes

: Fluoroarene complexes of the small bite angle bisphosphine Cy 2 PCH 2 PCy 2 (dcpm) have been prepared: [Rh(dcpm)( η 6 -1,2-F 2 C 6 H 4 )][Al{OC(CF 3 ) 3 } 4 ] and [Rh(dcpm)( η 6 -1,2,3-F 3 C 6 H 3 )][Al{OC(CF 3 ) 3 } 4 ]. These complexes act as precursors to a previously inaccessible σ -amine–borane complex [Rh(dcpm)( η 2 -H 3 B · NMe 3 )][Al{OC(CF 3 ) 3 } 4 ] of a small bite-angle bridging boryl- ene complex amine–borane de- hydrogenation/B–H activation. with Small Bite Amine–Borane


Results and Discussion
To avoid competition from zwitterion formation through coordination of [BAr F 4 ] -, the very weakly coordinating anion [Al{OC(CF 3 ) 3 } 4 ]was employed, the use of which has been pioneered and widely applied by Krossing. [25,26] F 4 ] slower partial conversion to 5 occurs over days, [28] and a single crystal X-ray structural determination confirmed its formulation (Scheme 2). To avoid such complications all future work was conducted exclusively with the [Al{OC(CF 3 ) 3 } 4 ]anion. 126 Hz). In the 19 F{ 1 H} NMR spectrum the fluoroarene resonances shift downfield upon complexation, with those of 2 observed at δ -146.7 (2 F) and -167.1 (1 F), relative to free 1,2,3-F 3 C 6 H 3 (δ -136.8, -163.5), as described previously for related systems. [29][30][31] Characterisation of 1 and 2 included a single-crystal X-ray crystallographic study (Scheme 2b for 2, supporting materials for 1), and complex 2 is the first structurally characterised example of an F 3 C 6 H 3 -transition metal complex. Significant disorder of the fluoroarene ring between different rotomers means that discussion of the geometric parameters is not appropriate, but the structure does demonstrate arene binding and the acute nature of the P-Rh-P angle [73.06(4)°]. The only previously reported example of 1,2,3-F 3 C 6 H 3 binding to a transition metal is [Rh(iBu 2 PCH 2 CH 2 PiBu 2 )(η 6 -1,2,3-F 3 C 6 H 3 )][BAr F 4 ], the characterisation of which was limited to in situ NMR spectroscopy and mass spectrometry as it is formed in equilibrium with its [BAr F 4 ]coordinated zwitterion. [11] Here, for 2, the combination of synthesis using concentrated solutions to overcome trace impurities and employing the very weakly coordinating anion [Al{OC(CF 3 ) 3 } 4 ]to obviate zwitterion formation allows for reliable access to such highly fluorinated arene complexes.
Having access to small bite angle bisphosphine complexes that were capable of binding amine-boranes, albeit made in situ, their ability to dehydrocouple H 3 B·NMe 2 H was evaluated, as we have previously shown that the P-Rh-P bite angle has an influence on the rate of this process. [7] The dehydrocoupling of H 3 B·NMe 2 H in 1,2,3-F 3 C 6 H 3 solvent was investigated using 5 mol-% 2 (Scheme 4  11 B NMR spectroscopy. [15,[32][33][34] In the 31 P{ 1 H} NMR spectrum only one major phosphorus-containing species was observed (7), as a complex second-order multiplet at δ = 55.9, hinting at the formation of a dimeric species. [35] A very broad resonance is observed in the 11 B NMR spectrum at δ = 59.0, with nothing observed to lower field. In the 1 H NMR spectrum two very well resolved multiplets were observed in the high field region at δ = -4.87 and -7.91, with relative integrals of 2:1 respectively, which do not sharpen upon 11  Crystalline material of complex 7 was obtained by recrystallisation from 1,2,3-F 3 C 6 H 3 /pentane. In the bulk this was always contaminated with a boron-containing species identified as the boronium salt [H 2 B(NMe 2 H) 2 ] + [δ( 11 B) = -2.0 ppm, J(BH) = 115 Hz; lit. δ( 11 B) = -2.8 ppm, J(BH) = 113 Hz], [32] but this did allow a single-crystal X-ray diffraction study to be performed, the results of which are shown in Figure 1. The solid-state structure shows a rearrangement of the bisphosphine ligands upon dimerisation, and complex 7 contains bridging dcpm ligand in an A-frame motif [37] and an aminoborylene BNMe 2 group. The {Rh(μ-dcpm)} 2 construct resembles that of other binuclear rhodium systems with similar ligands. [38,39] Although the hydride ligands were not located in the final Fourier difference map, the combination of NMR spectroscopic evidence and DFT studies (vide infra) confirm the presence of one bridging hydride trans-disposed to one terminal Rh-H at each Rh centre, (7). The geometry about each Rh is pseudosquare pyramidal, interestingly with a vacant coordination site trans to the borylene ligand. The cation has overall non-crystallographic C 2v symmetry. The Rh-B distances [2.015(6) and 1.983 (7) [36] but fall within the range seen for monomeric rhodium aminoboryl complexes of 2.034-1.929 Å. [10,41,42] [40] and the only structurally characterised μ-BNMe 2 example [{Mn(η 5 -C 5 H 5 )(CO) 2 } 2 (μ-BNMe 2 )] [1.39(1) Å]. [43] [36] However, the sharp signals observed for the hydrides in the 1 H NMR spectrum, that are unaffected by 11 B coupling, point to a bridging dihydrido aminoborylene motif, which would be expected to show lower field chemical shifts in the 11 B NMR spectra (>90 ppm), [40,44] although examples have been observed as far upfield as 74 ppm. [45] An obvious geometric distinction between a bridging aminoborane (μ-H 2 BNR 2 ) and a bridging aminoborylene dihydride (μ-BNR 2 ) structure is the orientation of the NR 2 moiety with respect to the RhBRh plane, as depicted in Figure 2. In the former case, e.g. I, a significant twist angle of 30.92°is observed between the RhBRh and HNH planes of I so as to maximise the orbital overlap between the B-H bonds and Rh centres. [36] This interaction is not present in 7 or [{Mn(η 5 -C 5 H 5 )(CO) 2 } 2 (μ-BNMe 2 )], [43] hence minimal twist angles are observed between the RhBRh and CNC planes of 7.25°and 8.38°, respectively. We postulate that the vacant coordination site trans to boron in complex 7 modifies the chemical shift to such an extent that the signal for the borylene is observed about 30 ppm to higher field than expected. Density functional theory calculations [46] in conjunction with Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) analyses have been employed to investigate the electronic structure of 7 (see Figure 3). Full geometry optimisation with the BP86 functional provided excellent agreement for the heavy atom positions and confirmed the presence of two terminal and one bridging hydride and square-pyramidal coordination around each Rh centre. Long H1···B1 and H2···B1 distances in excess of 2.9 Å preclude any direct bonding interaction and this is confirmed by the lack of a bond path between these centres (Figure 3a). In contrast, bond paths are computed between B1 and both Rh centres, as well as between Rh1/H1 and Rh2/H2. The associated bond critical points (BCPs) exhibit negative values of the total energy density H(r) and low ellipticities, ε, characteristics of σ-bonding that is predominantly covalent in nature. This contrasts with the μ-H 2 BNH 2 motif in I where the BCPs associated with the Rh-H and Rh-B bond paths have large ellipticities of about 0.5 au reflecting the anisotropic Bagostic Rh←H-B interaction. [36] The presence of the bridging hydride in 7 means that a ring critical point is seen between the Rh centres. The computed Rh1···Rh2 distance of 2.85 Å is in good agreement with the experimental value of 2.8266(5) Å. The lack of any Rh1···Rh2 interaction is confirmed in the NBO analysis which highlights three Rh-based (d-orbital) lone pairs, as well as Rh1-H1/Rh2-H2 and Rh1-B1/Rh2-B1 bonding orbitals. In contrast NBO calculations on [{Rh(η 5 -C 5 H 5 )(CO)} 2 {μ-BN-(SiMe 3 ) 2 }] clearly locate a Rh-Rh bonding orbital consistent with the presence of a metal-metal bond (see Supporting Information).
The formation of 7 is postulated to proceed in a similar manner to I and II (Scheme 5). [ [48,49] Such C-H activations are proposed to proceed via a cooperative mechanism wherein π-complexation of H 2 C=CRR′ to one metal enables σ-CH complexation at the other metal and consequently C-H cleavage. This bears parallels with the double B-H activation of transient H 2 B=NMe 2 observed here, although aminoboranes bind end-on rather than the side-on mode adopted by alkenes. [50,51] Aminoborane to aminoborylene transformations by double B-H activation of H 2 B=NR 2 (R = Cy, iPr) have been observed with mononuclear iridium and ruthenium complexes, [52,53] and related transformations on boranes are also known. [54,55] However, to the best of our knowledge the complete amine-borane to aminoborylene transformation is unprecedented, and represents a new method for the preparation of bridging borylenes.

Experimental Section
All manipulations, unless otherwise stated, were performed under an argon atmosphere using standard Schlenk line and glovebox techniques. Glassware was oven dried at 130°C overnight and flame dried under vacuum prior to use. Pentane and CH 2 Cl 2 were dried using a Grubbs type solvent purification system (MBraun SPS-800) and degassed by three successive freeze-pump-thaw cycles. 1,2-F 2 C 6 H 4 (purchased from Fluorochem, pretreated with alumina), 1,2,3-F 3 C 6 H 3 (purchased from Fluorochem, pretreated with alumina) and CD 2 Cl 2 were dried with CaH 2 , vacuum distilled and stored over 3 Å molecular sieves. H 3 B·NMe 3 and H 3 B·NMe 2 H were purchased from Sigma-Aldrich and sublimed prior to use. Li[Al{OC(CF 3 ) 3 } 4 ] [25] and [Rh(COD)Cl] 2 [56] were prepared by literature methods. All other chemicals were obtained from commercial sources and used as received.
[Rh(COD) 2 ][Al{OC(CF 3 ) 3 } 4 ]: Prepared according to the literature procedure for [Rh(COD) 2 ][BAr F 4 ]. [57] An orange solution of [Rh(COD)Cl] 2 (0.585 g, 1.19 mmol) and 1,5-cyclooctadiene (0.2 mL) in CH 2 Cl 2 (20 mL) was degassed by bubbling argon through the solution for 15 min. The solution was then added dropwise to a colourless slurry of Li[Al{OC(CF 3 ) 3 } 4 ] (2.31 g, 2.37 mmol) in CH 2 Cl 2 (60 mL) with vigorous stirring at ambient temperature. The colour of the slurry immediately changed to dark red. The reaction mixture was stirred at ambient temperature for a further 16 h and then filtered. The supernatant was then concentrated under vacuum (ca. 50 mL). Cooling to -20°C overnight afforded a red crystalline solid which was isolated by decanting, washed with pentane (2 × 2 mL) and dried under vacuum. Further concentration followed by cooling afforded a second crop. Yield (2.53 g, 83 %). 1

[Rh(dcpm)(COD)][Al{OC(CF 3 ) 3 } 4 ]:
Prepared according to the literature procedure for [Rh(dcpe)(COD)][BAr F 4 ]. [58] A solution of [Rh(COD) 2 ][Al{OC(CF 3 ) 3 } 4 ] (400.2 mg, 0.3111 mmol) in CH 2 Cl 2 (10 mL) was treated dropwise with a solution of dcpm (127.1 mg, 0.3111 mmol) in CH 2 Cl 2 (70 mL) at -78°C with vigorous stirring. Upon complete addition the colour of the reaction mixture changed from burgundy to orange. The reaction mixture was warmed to ambient temperature and stirred for 16 h. The solution was concentrated to 10 mL under vacuum and pentane (50 mL) was added to precipitate an orange solid which was isolated by filtration, washed with pentane (3 × 10 mL) and dried under vacuum. Yield 436.9 mg (0.2753 mmol, 89 %). The powder was then extracted into the minimum amount of CH 2 Cl 2 and layered with pentane, which afforded large orange crystals suitable for an X-ray diffraction study. 1  ] (100 mg, 63.0 μmol) was dissolved in 1,2-F 2 C 6 H 4 (5 mL) in a J. Young flask. The solution was freeze-pump-thaw degassed three times and refilled with H 2 (4 atm). The reaction mixture was stirred for 16 h, over which time the colour the solution changed from orange to yellow. Volatiles and excess H 2 were removed under vacuum and the resultant solid was washed with pentane (2 × 5 mL). The solid was extracted into the minimum volume of CH 2 Cl 2 , filtered and layered with pentane to afford yellow crystals suitable for X-ray diffraction which were isolated by filtration and dried under vacuum. Yield: 80 mg (49 μmol, 78 %). 1