Thermal Rearrangement via P–C Bond Cleavage of a Tridentate

Communication pubs.acs.org/Organometallics

Thermal Rearrangement via P−C Bond Cleavage of a Tridentate Diphosphine−N-Heterocyclic Carbene Ligand System Coordinated to Rhodium Bryan K. Shaw, Brian O. Patrick, and Michael D. Fryzuk* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 S Supporting Information *

ABSTRACT: The synthesis of a series of rhodium complexes, [PCP]RhX, where X = Cl, H, CH3 and PCP is a tridentate ligand that contains a central, saturated NHC donor flanked by two o-phenylenediisopropylphosphino groups, is described. These complexes were characterized by NMR spectroscopy (1H, 13C, and 31P) and, in the case of the X = Cl derivative, by X-ray crystallography. Investigation of the thermal reactivity of these complexes uncovered an unexpected ligand rearrangement process (for X = H, CH3) resulting from intramolecular P−C bond cleavage between one of the phosphine donors and the aryl linker of the ligand backbone. The structures of the P−C activation products are confirmed through X-ray diffraction analysis, and preliminary rate data in the case of the rhodium hydride are presented.

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temperatures, such rearrangements could help influence future ligand designs.34,35 Deprotonation of the iminium salt 1 with KN(SiMe3)2 generates the free NHC-containing PCP pincer 2, which upon addition to a suspension of [Rh(COD)Cl]2 in THF leads to the formation of the square-planar Rh(I) complex [PCP]RhCl (3), as indicated in Scheme 1.36,37 The doublet

n the two decades since the isolation of the first stable Nheterocyclic carbene (NHC),1,2 this class of compounds has been elevated from just chemical curiosities to important ancillary ligands in transition-metal catalysis.3−9 The use of NHCs as donor ligands is attractive, due to the relative ease with which the steric and electronic properties may be tuned through variation of the nitrogen substituents or alteration of the heterocycle.5,7,10 In addition, one can also incorporate NHC units into tridentate frameworks or pincer complexes with a variety of donor combinations, which include neutral [CNC],11−14 [PCP],15,16 and [NCN]17,18 as well as monoanionic [CCC]19−21 ligand sets. Metal complexes bearing pincer ligands are known to be quite robust, allowing for harsh reaction conditions without complex decomposition.22−25 For example, a palladium catalyst system supported by a [CNC] ligand consisting of a pyridine ring flanked by two NHC donors can withstand temperatures as high as 180 °C in air and remain catalytically active for use in the Heck reaction.12 We recently reported the synthesis of the tridentate ophenylene-bridged NHC diphosphine ligand precursor ([PCP]H)PF6 and its coordination chemistry with group 10 metals.26 Given the range of applications of non-NHC-derived pincer complexes of rhodium, including C−H bond activation,27−29 carbon dioxide reduction,30 and catalytic alkyne dimerization,31 in addition to the success of NHC-containing rhodium pincer complexes in catalysis,32,33 we investigated the coordination of our [PCP] ligand with rhodium. As elevated temperatures are often required in certain catalytic processes, the thermal stability of these rhodium complexes was examined, which resulted in the observation of an unexpected ligand rearrangement resulting from cleavage of one P−Caryl bond of the ligand backbone. As certain C−H activation processes involve anionic PCP ligand sets with group 9 metals at high © 2012 American Chemical Society

Scheme 1. Synthesis of [PCP]RhCl (3)

at δ 22.4 (1JRhP = 142.5 Hz) in the 31P{1H} NMR spectrum of 3 is typical for square-planar Rh(I) derivatives. Slow evaporation of a concentrated THF solution of 3 provided crystals suitable for X-ray diffraction analysis (Figure 1). Complex 3 has a slightly distorted square planar geometry similar to that of the cationic palladium and platinum [PCP] hydride cations synthesized previously.26 The central NHC ring of 3 is twisted above and below the plane defined by the NHC carbon and two phosphine donors (P1, C1, and P2) with Received: December 12, 2011 Published: January 18, 2012 783

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the analogous tridentate [PNP]RhCH3 complex that has been structurally characterized by X-ray crystallography.28 As already mentioned, we were interested in the thermal stability of these complexes. Interestingly, the valence isoelectronic group 10 hydride derivatives ([PCP]MH)PF6, where M = Ni, Pd, Pt, are thermally stable up to 120 °C in sealed tubes. Thus, we were surprised to discover that heating a solution of hydride 4 in C7D8 at 60 °C results in the disappearance of the starting material over 24 h, as evidenced by loss of the hydride signal. The 31P{1H} spectrum of the resulting product contains a pair of doublets of doublets (ABX pattern) at 53.7 ppm (JRhP = 97 Hz, JPP = 31 Hz) and 38.3 ppm (JRhP = 86 Hz, 2JPP = 31 Hz), which indicates two inequivalent phosphorus-31 nuclei cis coupled to each other and also coupled to 103Rh. Crystals suitable for X-ray diffraction could be obtained by slow evaporation of a concentrated THF solution of the product. The solid-state molecular structure of 6a confirms the loss of symmetry indicated by the NMR analysis, as a new rhodium complex resulting from intramolecular C−P bond activation of the PCP ligand is formed (Figure 2).

Figure 1. Solid-state molecular structure (ORTEP) of [PCP]RhCl (3). All hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−Rh1, 1.929(5); P1−Rh1, 2.3042(14); P2−Rh1, 2.2910(13); Rh1−Cl1, 2.4029(16); C1−N1, 1.370(6); C1−N2, 1.379(7); C1−Rh1−Cl1, 179.61(16); P1−Rh1− P2, 173.05(5); P1−Rh1−C1, 86.13(16); P1−Rh1−Cl1, 93.87(5).

torsion angles of 46.23 and 37.38°, respectively; the corresponding twists in the analogous palladium hydride cation ([PCP]PdH)PF6 are slightly less at 41.5 and 35.5°. In 3 the rhodium−carbon bond length (Rh1−C1) of 1.929(5) Å is relatively short in comparison to other RhI−CNHC bond lengths that have a mean average distance of 2.026 Å;38 this is likely attributable to the weak trans influence of the chloride ligand.39 [PCP]RhCl (3) serves as a versatile starting material, as shown in Scheme 2.40 Reaction of 3 with potassium Scheme 2

Figure 2. Solid-state molecular structure (ORTEP) of [PCC]Rh(HPPri2) (6a). All hydrogen atoms except H100 have been omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−Rh1, 1.976(2); C9−Rh1, 2.110(2); P1−Rh1, 2.2972(6); P2−Rh1, 2.2365(6); C1−N1, 1.345(3); C1−N2, 1.365(3); C1−Rh1−P1, 170.82(7); C9−Rh1−P2, 169.22(7); P1−Rh1−P2, 97.66(2); C9− Rh1−C1, 80.01(10); C9−Rh1−P1, 93.07(7).

To better understand the scope of this thermal process, the same conditions were applied to both the methyl complex 5 and the chloro derivative 3. Thermolysis of 5 at 60 °C results in the analogous P−C bond cleavage product [PCC]Rh(MePPri2) (6b). Diagnostic of this transformation is the observation of an ABX pattern in the 31P{1H} NMR spectrum, similar to that of 6a, and the observation of a doublet at δ 1.26 for the P−CH3 resonance in the 1H NMR spectrum. The solid-state molecular structure of 6b is shown in Figure 3; while the two solid-state structures (Figures 2 and 3) clearly show that both products result from aryl−P bond cleavage, and both products have common features, the main difference is the extent to which the monodentate phosphine deviates from the plane defined by C9, C1, and P2 for 6b. In the HPPri2 derivative 6a, the complex is square planar, while in 6b, the MePPri2 moiety is 0.97 Å above this plane. Also diagnostic is the C1−Rh1−P1 angle for the two complexes; in 6a, it is 170.82(7)°, while in 6b, this angle is 155.57(9)°. This distortion away from a square plane is likely a

triethylborohydride (KBEt3H) allows for the isolation of [PCP]RhH (4) in 77% yield. Most diagnostic for the squareplanar hydride is the doublet of triplets at −4.1 ppm (JRhH = 15 Hz, 2JHP = 22 Hz) in the 1H NMR spectrum.41 Reaction of 3 with a slight excess of methyllithium at 0 °C results in the formation of the square-planar methyl complex [PCP]RhCH3 (5) as a gray powder in 55% isolated yield (Scheme 2). In the 1H NMR spectrum, the Rh−CH3 protons appear at δ 0.55 as a doublet of triplets (2JRhH = 1.5 Hz, 3JHP = 6.5 Hz), and in the 13C{1H} NMR spectrum, the methyl carbon resonance (doublet of triplets) is observed at −10.3 ppm (1JRhC = 19 Hz, 2JPC = 13 Hz). These values are similar to those for 784

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not been reported. Interestingly, reversible cleavage of an arylsilyl linkage has been reported for a tridentate PSiP donor set.47 We have obtained some very preliminary rate data for this process by following the rate by which the starting complex disappears via 31P{1H} NMR spectroscopy. The rhodium hydride 4 rearranges to 6a at an average rate constant of k = 4.7 × 10−5 s −1 with a half-life of 3.9 h (at 60 °C) that is independent of concentration, indicating first-order reaction kinetics. Unfortunately, the conversion of methyl 5 to 6b does not proceed cleanly enough to obtain reliable kinetic data. However, the ligand rearrangement reaction of 5 to 6b proceeds in about half the time (12 h) required to convert the hydride 4 to 6a (22 h). While the data are preliminary, they suggest the ability to alleviate steric repulsion between the isopropyl groups and the ligand trans to the NHC lowers the barrier to this process. Why the chloro complex 3 is not prone to this rearrangement may be due to the intrinsic electronwithdrawing nature of a chloride ligand that makes the rhodium center in 3 less electron rich than in either 4 or 5, which in turn would reduce the tendency for the oxidative addition of the P− Caryl unit. The facile thermal P−C bond-breaking process for these neutral Rh(I) PCP derivatives described in this paper is surprising, in light of the stability of the isovalent Pd(II) cationic hydride ([PCP]PdH)PF6 to this rearrangement under even more forcing conditions. This particular observation may be due to the fact that the cationic palladium complex is more electrophilic than its neutral counterpart and therefore less electron rich, which would further impede the oxidative addition step. Given the aforementioned observation that no rearrangement occurs for the chloro derivative 3, it is clear that electronic factors rather than steric factors are at play here.

Figure 3. Solid-state molecular structure (ORTEP) of [PCC]Rh(MePPri2) (6b). All hydrogen atoms except those on C28 have been omitted for clarity. Selected bond lengths (Å) and bond angles (deg): C1−Rh1, 1.966(3); C9−Rh1, 2.097(3); Rh1−P1, 2.3321(9); Rh1− P2, 2.2610(9); C1−N1, 1.356(4); C1−N2, 1.368(4); C1−Rh1−P1, 155.57(9); C9−Rh1−P2, 167.66(9); P1−Rh1−P2, 102.48(3); C9− Rh1−C1, 79.45(13); C9−Rh1−P1, 89.87(9).

result of the larger MePPri2 moiety in comparison to HPPri2. The implications of the steric restrictions created by the isopropyl groups on the phosphine atoms have been detailed in an earlier report.46 In contrast to the above facile transformations, thermolysis of the chloro derivative 3 did not result in any observable rearrangement even up to 110 °C; only recovered starting material was obtained. A reasonable mechanism for formation of the aforementioned thermal rearrangement products is outlined in Scheme 3. Upon thermolysis, oxidative addition of

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Scheme 3. Mechanistic Proposal for the Thermal Rearrangement of [PCP]RhR to [PCC]Rh(RPPri2) (R = H, CH3)

ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and CIF files giving details of the syntheses, characterization data, and X-ray crystal structure data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1 604 822-8710. Tel: +1 604 822-2471. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded by a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada.

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one of the aryl−-P bonds leads to the formation of a coordinatively unsaturated Rh(III) phosphido−aryl species, which subsequently undergoes reductive elimination to generate the observed product. Transition-metal-mediated C−P bond activation has previously been observed experimentally42−45 and is often a deactivation pathway for arylphosphine moieties, although to our knowledge, the occurrence of this kind of process in a tridentate ligand has

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