One‐Electron Oxidation of [M(P^tBu₃)₂] (M=Pd, Pt): Isolation of Monomeric [Pd(P^tBu₃)₂]⁺ and Redox‐Promoted C−H Bond Cyclometalation

Abstract

Oxidation of zero‐valent phosphine complexes [M(P^tBu₃)₂] (M=Pd, Pt) has been investigated in 1,2‐difluorobenzene solution using cyclic voltammetry and subsequently using the ferrocenium cation as a chemical redox agent. In the case of palladium, a mononuclear paramagnetic Pd^I derivative was readily isolated from solution and fully characterized (EPR, X‐ray crystallography). While in situ electrochemical measurements are consistent with initial one‐electron oxidation, the heavier congener undergoes C−H bond cyclometalation and ultimately affords the 14 valence‐electron Pt^II complex [Pt(κ ² _PC‐P^tBu₂CMe₂CH₂)(P^tBu₃)]⁺ with concomitant formation of [Pt(P^tBu₃)₂H]⁺.

Over the past few decades a rich variety of chemistry has emerged based on the reactions of palladium and platinum complexes in the 0 and +II formal oxidation states, epitomized by the omnipresence of palladium catalyzed cross‐coupling reactions in contemporary organic chemistry.1, 2 In contrast, the organometallic chemistry of well‐defined complexes of these elements bearing formal +I oxidation states is much less established and examples are largely limited to unstable or dinuclear species with distinct metal–metal bonds.3, 4 Halogen bridged palladium complexes of the type [Pd(μ‐X)(P^tBu₃)]₂ (X=Br, I) are notable examples and are believed to act as reservoirs for reactive {Pd⁰(P^tBu₃)} fragments in catalytic transformations.5 In other systems, Pd^I and Pt^I species have been postulated as intermediates, but with little supporting evidence.6 With a view to isolating well‐defined mononuclear complexes in the +I oxidation state relevant to catalysis, we report herein our work involving one‐electron oxidation of widely used and commercially available palladium(0) and platinum(0) complexes of tri‐tert‐butylphosphine [M⁰(P^tBu₃)₂] (M=Pd, 1 a; Pt, 1 b).

As a starting point we determined the redox potentials of 1 a and 1 b by cyclic voltammetry (CV) in the weakly coordinating solvent 1,2‐difluorobenzene (0.2 m [ⁿBu₄N][PF₆] electrolyte, Figure 1).7 Reversible one‐electron oxidation was observed at E _1/2=−0.44 V (1 a) and E _1/2=−0.10 V (1 b) relative to Fc/[Fc]⁺ (Fc=ferrocene). The electrochemical characteristics of closely related cyclic alkyl(amino) carbene (CAAC) analogues have recently been studied by CV and the redox potentials of 1 a and 1 b are similar in magnitude to those found for [M⁰(CAAC)₂] (M=Pd, −0.60 V; Pt, −0.07 V) in THF (0.1 m [ⁿBu₄N][ClO₄]).8 Consistent with the generation of a stable Pd^I species (2 a), the peak current ratios (i _p ^red/i _p ^ox) in the palladium voltammograms are essentially unity (ca. 0.99). Conspicuously lower ratios were observed for the platinum complex (ca. 0.90).

Figure 1.

Cyclic voltammograms for the oxidation of 1 a and 1 b in 1,2‐C₆H₄F₂ (2 mm 1; 0.2 m [ⁿBu₄N][PF₆] electrolyte; glassy carbon working electrode, Pt counter electrode and Ag wire reference electrode; scan rates=10, 30, 50, 70, and 100 mV s⁻¹).

Encouraged by these data, 1 a was reacted with one equiv of [Fc][PF₆] in 1,2‐difluorobenzene at 293 K and dark blue [Pd^I(P^tBu₃)₂][PF₆] 2 a was subsequently isolated in 92 % yield following addition of n‐pentane. The electrochemical characteristics of isolated 2 a are equivalent to those measured in situ starting from 1 a (E _1/2=−0.42 V; see Supporting Information). This new paramagnetic species was additionally characterized in solution using UV/Vis spectroscopy (λ _max=667 nm), ESI‐HRMS (positive ion mode, 510.2736 m/z [M]⁺; calculated 510.2740 m/z), and EPR spectroscopy. The EPR spectrum (1,2‐C₆H₄F₂ glass at 200 K, Figure 2), shows a superposition of a single resonance at g=2.316(5) with a lower intensity sextet arising from hyperfine coupling to ¹⁰⁵Pd (I=5/2, 22 % abundance), corroborating formation of an S=1/2 Pd^I species. The unusually large ¹⁰⁵Pd hyperfine coupling of approximately 25 mT, and lack of resolved coupling to ³¹P (I=1/2, 100 % abundance) is consistent with strong localization of the unpaired electron spin on the Pd center. Complex 2 a crystallizes in the high‐symmetry cubic space group Pa $\bar{3}$ with the palladium atom on a center of inversion (Figure 2). In comparison to 1 a, the Pd−P bond length is significantly elongated, from 2.285(3) to 2.3469(6) Å (Δ(Pd−P)=+0.062(4) Å); the P‐Pd‐P angles in both cases are symmetry enforced at 180°.9 To the best of our knowledge, this is the first example of an unsupported two‐coordinate Pd^I complex. A similar bond length elongation has been noted in closely related NHC complexes of Ni⁰/Ni^I (Δ(Ni−C)=+0.08(2) Å).10 Isolated 2 a is air‐sensitive in solution, but shows good stability under an argon atmosphere. For instance, under argon the EPR spectrum intensity was essentially unchanged after 24 h at 293 K (15 mm). However, slow degradation of 2 a was observed by UV/Vis spectroscopy under high dilution conditions (t _1/2≈30 h; 0.15 mm), which we attribute to the presence of adventitious water as the rate of degradation increased significantly when water was added deliberately. Moreover, 2 a can be stored in the solid‐state in air (72 h) with no evident change by UV/Vis spectroscopy.

Figure 2.

The solid‐state structure22 and EPR spectrum of 2 a (1,2‐C₆H₄F₂ glass, 200 K, a.u.=arbitrary units).11 Ellipsoids are set at 50 % probability; anion omitted for clarity. The starred atom is generated by the symmetry operation 1−x, 1−y, 1−z. Selected data: Pd1−P2 2.3470(6) Å; P2‐Pd1‐P2* 180°, Pd1‐P2‐C3 108.81(5)°.

When preparation of the analogous Pt^I complex 2 b was attempted by reaction of 1 b with one equiv of [Fc][PF₆], a 1:1 mixture of the new diamagnetic cyclometalated complex [Pt^II(κ ² _PC‐P^tBu₂CMe₂CH₂)(P^tBu₃)][PF₆] 3 b and known Pt^II hydride [Pt^II(P^tBu₃)₂H][PF₆] 4 (δ(¹H) −36.30 ppm; ² J _PH=8.6, ¹ J _PtH=2590 Hz; δ(³¹P) 86.3 ppm; ¹ J _PtP=2621 Hz) was formed within 15 min instead, as indicated by ¹H and ³¹P NMR spectroscopy (Fc observed; Scheme 1).12 This outcome suggests only transient stability of 2 b in solution, with subsequent C−H bond homolysis accounting for the divergence from fully reversible one‐electron oxidation of 1 b observed by CV.13 Reaction of 1 b with two equiv of [Fc][PF₆] in the presence of excess hindered base 2,6‐bis(decyl)pyridine (5 equiv), which is able to deprotonate 4, resulted in selective formation of 3 b within 15 min. In this manner, 3 b was isolated in 93 % yield following successive crystallizations from 1,2‐C₆H₄F₂ to remove ferrocene, excess base, and pyridinium salt.13 For comparison, no significant reaction was detected by ¹H or ³¹P NMR spectroscopy on mixing of 1 b and 2,6‐bis(decyl)pyridine in 1,2‐difluorobenzene at 293 K (24 h) or heating 1 b alone in 1,2‐difluorobenzene at 353 K (24 h).

Scheme 1.

Chemical oxidation of 1 b.

Two independent but structurally similar cations are observed in the solid‐state structure of 3 b (one is shown in Figure 3), both illustrating adoption of a T‐shaped coordination geometry14 and cyclometalation of one of the tert‐butyl substituents; these are identified by distinctly acute Pt1‐P2‐C3 angles [90.0(3)/89.5(3)°] and Pt1−C4 bond lengths of 2.063(17)/2.065(17) Å. The 3 b cation is formally a 14 valence‐electron (VE) complex, but is stabilized by adoption of an agostic interaction between the non‐cyclometalated phosphine ligand and Pt center (Pt1⋅⋅⋅C4B 2.83(2)/2.84(2) Å). In solution, the structure of 3 b was fully corroborated by NMR spectroscopy (CD₂Cl₂, 298 K). Formation of the metallacycle is apparent by distinctive ¹H and ¹³C methylene resonances at δ(¹H) 2.75 ppm (² J _PtH=110 Hz) and δ(¹³C) 10.3 ppm (¹ J _PtC=670 Hz) with platinum satellites, two doublet ³¹P resonances with a large (trans) ² J _PP coupling constant and platinum satellites (δ(³¹P) 59.1 ppm (¹ J _PtP=2896 Hz, ² J _PP=317 Hz, P ^tBu₃), δ(³¹P) 25.2 ppm (¹ J _PtP=1916 Hz, ² J _PP=317 Hz, P ^tBu₂CMe₂CH₂)), and a platinum chemical shift of δ(¹⁹⁵Pt) −3816 ppm (225 K). Although the signals associated with the non‐cyclometalated phosphine ligand broadened on cooling to 185 K, the agostic interaction could not be definitively resolved by ¹H NMR spectroscopy.

Figure 3.

Solid‐state structures of 3 b and 6.22 Ellipsoids are set at 50 % and 30 % probability, respectively; minor disordered components and anions omitted for clarity; only one of the two independent molecules is shown for 3 b. Starred atoms in 3 b are generated by the symmetry operation 1−x, 2−y, 1−z. Selected data 3 b: Pt1−P2 2.297(2) Å, Pt1−C4 2.063(17) Å, Pt1⋅⋅⋅C4B* 2.83(2) Å; P2‐Pt1‐P2* 180°, Pt1‐P2‐C3/C3* 90.0(3)°. 6: Pt1−P2 2.235(2) Å, Pt1−C4 2.077(10) Å, Pt1−N15 2.080(7) Å, Pt1−N26 2.156(7) Å; P2‐Pt1‐N15 166.3(2)°, C4‐Pt1‐N26 175.4(3)°, Pt1‐P2‐C3 88.5(3)°.

Cyclometalation reactions of Pt^II complexes have extensive precedent.14a, 15 For instance, T‐shaped complexes [Pt^II(κ ² _PC‐PR₂C₆H₃MeCH₂)(PR₂Xyl)]⁺ (R=Cy, Ph; Xyl = 2,6‐dimethylphenyl) with similar structural and spectroscopic metrics compared to 3 b, were prepared by cyclometalation reactions involving halide abstraction from [Pt^II(PR₂Xyl)₂(Me)Cl] and subsequent elimination of methane.16 Intramolecular C−H bond activation of P^tBu₃ in [Pt^II(P^tBu₃)₂HX] (X=Cl, Br, I, OTf, NO₂) has also been described and results in coordinatively saturated products [Pt^II(κ ² _PC‐P^tBu₂CMe₂CH₂)(P^tBu₃)X].17 In the case of 3 b, the presence of a Pt^II intermediate proceeding cyclometalation can be discounted on the basis of the electrochemical characteristics of 1 b. Instead the formation of 3 b and 4 presumably occurs via concerted bimetallic (radical) oxidative addition,18 or proceeds through a common Pt^III alkyl hydride intermediate [Pt^II(κ ² _PC‐P^tBu₂CMe₂CH₂)(P^tBu₃)H]⁺ (5). In the latter case, subsequent comproportionation (5+2 b), disproportion (via a Pt^IV alkyl dihydride), or Pt−H bond homolysis (i.e. 2×5→2×3 b+H₂; 3 b+H₂→4) would afford the observed 1:1 mixture of 3 b and 4.19

Seeking to gain more insight into this mechanism, trapping of the postulated intermediate 5 was attempted by coordination of 2,2′‐bipyridine (bipy). However, oxidation of 1 b with either one or two equiv of [Fc][PF₆] in the presence of one equiv of bipy resulted in formation of a new cyclometalated complex [Pt^II(κ ² _PC‐P^tBu₂CMe₂CH₂)(bipy)][PF₆] 6 instead, alongside protonated phosphine (δ(³¹P) 54.2 ppm). The identity of this new complex was verified by independent synthesis from 3 b and bipy in 1,2‐C₆H₄F₂ (97 % yield of isolated product). As with 3 b, the cyclometalated phosphine in 6 is characterized by an acute Pt1‐P2‐C3 angle (88.5(3)°) and bears a similar Pt1−C4 bond length of 2.077(10) Å (Figure 3). Moreover, both solution and solid‐state data are fully consistent with a coordinatively saturated metal complex. Notably, the substantially higher trans‐influence of the methylene ligand is reflected in different Pt−N bond lengths (Pt1−N15, 2.156(7) versus Pt1−N26, 2.080(7) Å); the associated ¹³C resonance shows a reduced ¹ J _PtC coupling in comparison to 3 b (580 versus 670 Hz). Stronger Pt−P bonding is apparent in 6 relative to 3 b, on the basis of a shorter Pt−P bond (2.235(2) versus 2.297(2)/2.299(3) Å), and a larger ¹ J _PtP coupling constant determined by ³¹P NMR spectroscopy (3105 versus 1916 Hz). A platinum chemical shift of δ(¹⁹⁵Pt) −3788 ppm (225 K) was also measured for 6 and is very similar to that of 3 b (δ(¹⁹⁵Pt) −3816 ppm).

Reaction of isolated 3 b with H₂ (1 atm) results directly in the formation of 4, which is reconcilable with Pt−H bond homolysis or disproportion (via an unstable Pt^IV alkyl dihydride intermediate) during the formation of 3 b/4. However, the underlying mechanism is still not completely clear at this time. For instance, we cannot discount the formation of 3 b through a pathway involving deprotonation of 5 (mediated by 1 b 20 or 2,6‐bis(decyl)pyridine) and a second one‐electron oxidation. The redox potential of the associated Pt^I/Pt^II couple, assessed by CV experiments using both isolated 3 b (E _1/2=−1.90 V, irreversible) and 6 (E _1/2=−1.68 V, i _p ^ox/i _p ^red≈0.96), indicates that such a one‐electron oxidation is at least conceptually feasible using [Fc][PF₆] (see Supporting Information for CVs).

Motivated by the cyclometalation observed on oxidation of 1 b, we have also preliminarily investigated whether similar reactivity can be induced in the palladium analogue. Our studies are on‐going, but we do note that reaction of 1 a with two equiv of [Fc][PF₆] in the presence of excess 2,6‐bis(decyl)pyridine (5 equiv) resulted in the gradual appearance of a diamagnetic complex with spectroscopic characteristics consistent with cyclometalation (3 a; δ(³¹P) 57.0, −1.3 ppm; ² J _PP=316 Hz).21 However, this species was only formed in situ in about 30 % yield after 72 h at 293 K, as measured by NMR spectroscopy (using an internal standard), and the resulting reaction mixture has proved intractable so far to further characterization.

In summary, we have described a simple method for accessing the reaction chemistry of mononuclear palladium and platinum complexes bearing a +I formal oxidation state, as demonstrated by one‐electron oxidation of [M⁰(P^tBu₃)₂] (M=Pd, Pt) using [Fc][PF₆]. While the Pd^I derivative was readily isolated from solution and fully characterized, the heavier congener undergoes C−H bond cyclometalation to afford the 14 VE Pt^II complex [Pt^II(κ ² _PC‐P^tBu₂CMe₂CH₂)(P^tBu₃)]⁺ with concomitant formation of [Pt^II(P^tBu₃)₂H]⁺. Future work is focused on charting the reactivity and catalytic activity of these novel Group 10 species, and will be published in due course.

Supporting information

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Supplementary

T. Troadec, S.-y. Tan, C. J. Wedge, J. P. Rourke, P. R. Unwin, A. B. Chaplin, Angew. Chem. Int. Ed. 2016, 55, 3754.