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Insertion of a Nontrigonal Phosphorus Ligand Into a Transition Metal-Hydride: Direct Access to a Metallohydrophosphorane

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Insertion of a Nontrigonal Phosphorus Ligand Into a Transition Metal-Hydride: Direct Access to a Metallohydrophosphorane Insertion of a Nontrigonal Phosphorus Ligand into a Transition Metal-Hydride: Direct Access to a Metallohydrophosphorane The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Tanushi, Akira and Alexander T. Radosevich. "Insertion of a Nontrigonal Phosphorus Ligand into a Transition Metal-Hydride: Direct Access to a Metallohydrophosphorane." Journal of the American Chemical Society 140, 26 (June 2018): 8114-8118 © 2018 American Chemical Society As Published http://dx.doi.org/10.1021/jacs.8b05156 Publisher American Chemical Society (ACS) Version Author's final manuscript Citable link https://hdl.handle.net/1721.1/123455 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. HHS Public Access Author manuscript Author ManuscriptAuthor Manuscript Author J Am Chem Manuscript Author Soc. Author Manuscript Author manuscript; available in PMC 2018 July 06. Published in final edited form as: J Am Chem Soc. 2018 July 05; 140(26): 8114–8118. doi:10.1021/jacs.8b05156. Insertion of a Nontrigonal Phosphorus Ligand into a Transition Metal-Hydride: Direct Access to a Metallohydrophosphorane Akira Tanushi and Alexander T. Radosevich* Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 Abstract The synthesis and reactivity of an NPN-chelating ligand containing a nontrigonal phosphorous triamide center (L1 = P(N(o-N(2-pyridyl)C6H4)2) is reported. Metalation of L1 with RuCl2(PPh3)3 gives RuCl2(PPh3)(L1) (2). By contrast, metalation of L1 with RuHCl(CO)(PPh3)3 H yields RuCl(CO)(PPh3)(L1 ) (3), a chelated 10-P-5 ruthenahydridophosphorane, via net insertion into the Ru–H bond. Hydride abstraction from 3 with Ph3CPF6 gives [RuCl(CO)(PPh3)(L1)]PF6 (4); reaction of 4 with NaBH4 returns 3. TOC graphic Tricoordinate phosphorus (8-P-3) compounds 1, 2 are essential donor ligands in transition metal chemistry. 3-6 Within typical (pseudo)threefold symmetry, the Lewis basicity and nucleophilicity of trigonal pyramidal 8-P-3 compounds makes the mixture of a transition metal fragment (M–X) and a phosphine (R3P) a robust and reliable route to the formation of R3P–M–X complexes (Figure 1, left). Contrarily, we describe in this Communication a noncanonical ligation event arising from metalation of a nontrigonal 8-P-3 compound. Specifically, we report the insertion of a distorted 8-P-3 phosphorous triamide center into a transition metal-hydride bond to form a 10-P-5 metallophosphorane7-31 (Figure 1, right) in preference to the typical 8-P-4 complex. Through these results, we realize the merger of phosphorus-based biphilic reactivity 32 with transition metal ligancy and thereby provide a *Corresponding Author: [email protected]. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental procedures, spectral data (1H, 13C, 31P NMR), cartesian coordinates (.pdf) Crystallographic files (.cif) The authors declare no competing financial interests. Tanushi and Radosevich Page 2 new entry point for the study of supporting ligands based on higher coordination number Author ManuscriptAuthor Manuscript Author Manuscript Author Manuscript Author phosphorus fragments. Nontrigonal tricoordinate phosphorus compounds (e.g. molecular symmetries C2v, Cs) 33 - 37 express enhanced biphilic reactivity. For instance, Cs-symmetric phosphorous triamide 1 inserts into E–H bonds (E = −OR, −NHR),38,39 in some cases reversibly. This and related P-centered reactivity have been applied to a variety of stoichiometric and catalytic chemical transformations.40 In pursuit of transition metal complexation, we elected to embed the 3 41 nontrigonal phosphorous triamide PN3 core within a κ -chelating framework in order to afford kinetically stable complexes. To this end, ligand L1 was designed to incorporate two 2-pyridyl moieties flanking the phosphorous triamide PN3 core. Its synthesis was accomplished in two steps from bis(2-aminophenyl)amine 42 by twofold electrophilic heteroarylation with 2-chloropyridine, followed by reaction with PCl3 in the presence of NEt3 (Scheme 1). In the solution phase, 1H NMR spectra of L1 exhibit a twofold equivalence of resonances (δ 31 1 8.26–6.92 ppm), indicating a time-averaged Cs molecular symmetry or higher. A P{ H} NMR spectrum of L1 displays a singlet at δ 141.7 ppm (Figure 2a), residing upfield as compared to 1 (δ 159.8 ppm). Structural analysis suggests the origin of this spectral distinction. X-ray diffraction of a colorless single crystal of the ligand L1 revealed a solid state structure of approximate Cs local symmetry (Figure 3a) marked by folding of the phosphorous triamide moiety along the P1–N1 axis, in line with previous observations for 1. However, in contrast to 1, the ∠N2–P1–N3 angle for L1 is decidedly more acute (cf. 108.67(4)° vs. 115.21(7)°) while the bond lengths d(P1–N2/3) are somewhat longer (Table 1). These structural features can be rationalized qualitatively by reference to Bent’s rule,43 where the electron-withdrawing 2-pyridyl N-substituents attract phosphorus p-character to the P1–N2/3 bonds, resulting in elongated d(P–N2/3) distances and contracted ∠N2–P1–N3 angles. The complementary accrual of s-character in the nonbonding P-based lone pair therefore increases nuclear shielding for L1 as observed in the 31P{1H} NMR chemical shift. 44 Metalation of L1 by treatment with RuCl2(PPh3)3 was realized in THF at ambient 31 1 temperature to afford an octahedral complex RuCl2(PPh3)(L1) (2). In the resulting P{ H} NMR spectra, the resonance corresponding to the phosphorus nucleus of L1 is found downfield (relative to free ligand L1) as a doublet at δ 186.8 ppm with J = 49 Hz; the corresponding coupling partner arising from the phosphorus nucleus of PPh3 resonates at δ 42.7 ppm with complementary J = 49 Hz coupling (Figure 2b). The κ3-chelating mode is confirmed by X-ray diffraction of a single crystal sample, where the facial coordination of the tridentate L1 ligand is evident (Figure 3b). The Ph3P ligand is found cis with respect to the phosphorus atom in the L1 fragment, giving a complex of overall C1 symmetry. The high number of complex overlapping resonances in the aryl region of the solution phase 1H NMR spectra for 2 are congruent with the elimination of the symmetry element about the central P1–N1 axis of L1 upon ligation. The ∠N2–P1–N3 angle is broadened (113.94(4°) as compared to free L1, braced by the constraint imposed by the chelation of the pyridyl arms to Ru. A distinctive feature of the structure for 2 is the exceptionally short d(Ru1–P1) bond distance (2.1262(3) Å), which is significantly shorter than the typical range for R3P–Ru J Am Chem Soc. Author manuscript; available in PMC 2018 July 06. Tanushi and Radosevich Page 3 bonds (2.20 Å < d < 2.45 Å) and only modestly longer than the shortest characterized R3P– Author ManuscriptAuthor Manuscript Author Manuscript Author Manuscript Author Ru distance (2.0883(6) Å),45 found for a NPN κ3-chelate containing a central tricoordinate phosphorus site substituted with electronegative heteroatoms similar to 2. The electronegative substituents probably contribute to the short Ru–P length of 2 by promoting π-backbonding from the metal.46 47 Metalation of L1 with RuHCl(CO)(PPh3)3 was similarly attempted; however, the expected congener of 2 was not returned. Instead, 31P{1H} NMR spectra (Figure 2c) showed formation of a new complex with a large upfield shift for the phosphorus atom of L1 at δ −12.3 ppm and coupling (doublet, J = 29 Hz) to one remaining PPh3 ligand (δ 54.7 ppm, 2 1 31 JP-P = 29 Hz). In the H-coupled P NMR spectrum, the signal at δ −12.3 ppm is further split into a doublet with a large coupling constant (J = 535 Hz). The magnitude of this coupling constant suggests the formation of a direct P–H bond. Indeed, the corresponding proton coupling partner is found in the 1H NMR spectrum as a doublet at δ 7.66 ppm (J = 535 Hz), well downfield from the typical range for a Ru(II)–H (i.e. δ < 0 ppm). The formation of P–H bond is also supported by an absorption at 2226 cm−1 in the IR spectrum, which can be assigned to a P–H stretching mode. Together, these spectra suggest that reaction of L1 with RuHCl(CO)(PPh3)3 affords a stable complex in which the hydrido ligand of Ru is transferred to the phosphorus atom, which we formulate as RuCl(CO)(PPh3) (L1H) (3, Scheme 1). The solid-state structure of 3, determined by X-ray diffraction on a single crystal confirms this assignment (Figure 3c). Indeed, the 10-P-5 nature of the phosphorus environment is 48 evident. The geometry about P is described by a distorted trigonal bipyramid (τ5 = 0.50) with the H and Ru substituents occupying equatorial positions; by consequence, the phosphorous triamide framework is greatly planarized as compared to 2 with the 2- pyridylamides occupying the trans diapical sites (∠N2–P1–N3 = 170.74(7)°). The resulting trigonal bipyramidal geometry contains two electronegative N atoms in diapical positions and electropositive H and Ru atoms in two equatorial positions, which follows a general 49 trend of phosphoranes. The ruthenium-phosphorus bond distance (d(Ru1–P1) = 2.2509(4) Å) is ca. 0.12 Å longer than in 2. The origin of this change is given at least in part by the ring constraints imposed by the chelating pyridyl arms, which draw the pentacoordinate phosphorus away from an idealized axial octahedral coordination site about ruthenium (compare ∠P1–Ru–Cl1 = 168.85(1)° vs.
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