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Phosphinidene Reactivity of a Transient Vanadium P#N Complex Marc-André Courtemanche, Wesley J. Transue, and Christopher C. Cummins J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b10545 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016
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Phosphinidene Reactivity of a Transient Vanadium P≡N Complex Marc-Andr´e Courtemanche, Wesley J. Transue, and Christopher C. Cummins∗ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received November 22, 2016; E-mail:
[email protected] ArtBuN
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Abstract: Towards the preparation of a coordination complex of the heterodiatomic molecule PN, P≡N-V(N[t Bu]Ar)3 (1, Ar = 3,5-Me2 C6 H3 ), we report the use of ClPA (A = C14 H10 , anthracene) as a formal source of phosphorus(I) in its reaction with Na[NV(N[t Bu]Ar)3 ] (Na[4]) to yield trimeric cyclo-triphosphane [PNV(N[t Bu]Ar)3 ]3 (3) with a core composed exclusively of phosphorus and nitrogen. In the presence of NapS2 (peri-1,8-naphthalene disulfide), NapS2 P– NV(N[t Bu]Ar)3 (6) is instead generated in 80% yield, suggesting trapping of transient 1. Upon mild heating, 3 readily fragments into dimeric [PNV(N[t Bu]Ar)3 ]2 (2), while in the presence of bis(trimethylsilyl)acetylene or cis-4-octene, the respective phosphirene (Ar[t Bu]N)3 VN–PC2 (SiMe3 )2 (7) or phosphirane (Ar[t Bu]N)3 VN–P(C8 H16 ) (8) compounds are generated. Kinetic data were found to be consistent with unimolecular decay of 3 and [2+1] cycloaddition with radical clocks ruled out a triplet intermediate, consistent with intermediate 1 reacting as a singlet phosphinidene. In addition, both 7 and 8 were shown to reversibly bind cis-4-octene and bis(trimethylsilyl)acetylene, serving as formal sources of 1, a reactivity manifold traditionally reserved for transition-metals.
The state of P≡N solution chemistry contrasts sharply with that of homodiatomic molecules N2 and P2 . Indeed, dinitrogen reactivity has been extensively investigated 1 and new synthetic routes have recently enabled the exploration of the rich chemistry of diphosphorus. 2–4 Despite longstanding spectroscopic interest 5 and recent efforts in the development of molecular precursors to PN, 6,7 the chemistry of this interstellar species remains virtually unexplored. While there are rare examples of main group molecules containing lowcoordinate phosphorus-nitrogen units such as iminophosphines 8 and related species, 9 PN lacks even coordination complexes. 10 Presumably, the lack of reports on PN chemistry is tied to the difficulty of accessing an appropriate synthetic precursor. Just so, our group has had longstanding interest in anthracene-based molecular precursors as a platform for the delivery of reactive small fragments upon mild heating. In this vein, precursors to phosphinidenes, 6 P2 , 4 and HCP 11 have allowed their access under conditions amenable to organic and organometallic synthesis. We aimed to leverage this anthracene delivery platform to generate a PN complex, bridging the chemistry of small molecules and of low-valent early transition metals. We hypothesized that V(N[t Bu]Ar)3 (Ar = 3,5-Me2 C6 H3 ) 12 could provide a protective pocket to accommodate PN within its sterically congested anilide ligand environment. Herein, in pursuit of a terminal complex of PN (1), we report dimeric diphosphene (2) and trimeric cyclotriphosphane (3), the latter serving as a synthetic equivalent of 1 under mild conditions (Figure 1).
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Figure 1. Molecular depiction of monomeric (1), dimeric (2) and trimeric (3) complexes of PN.
Formation of the key P–N bond was envisioned to occur through treatment of the previously reported Na[NV(N[t Bu]Ar)3 ] (Na[4]) 13 with ClPA 4 (A = C14 H10 , anthracene), generating 5 en route to 1 (Scheme 1). Subsequent anthracene elimination from 5 would then produce 1 when provided with sufficient thermal energy. Interestingly, when the reaction was carried out in thawing THF, the dark red solids that were isolated from the reaction mixture did not show a characteristic signal for the bridgehead protons of a 7-phosphanorbornadiene framework by 1 H NMR spectroscopy, indicating 5 was not the isolated product. This was confirmed by 51 V NMR spectroscopy, which showed broad resonances centered at 212 (∆ν1/2 2100 Hz) and 77 ppm (∆ν1/2 810 Hz), integrating to a 2:1 ratio. An identical pattern was observed in the 31 P{1 H} NMR spectrum of the product, which exhibited two broad resonances with a 2:1 integration at 201 (∆ν1/2 1040 Hz) and 112 ppm (∆ν1/2 760 Hz). Conclusive identification of this product as trimeric 3 was facilitated by X-ray crystallography, revealing a cyclo-triphosphane moiety with monomeric units oriented in a two-up-one-down conformation. The V–N distance of 1.698(2) ˚ A was similar to other related species, 14,15 while the average P–N bond distance of 1.754 ˚ A was consistent with a short single bond. 13,16 Despite the conspicuous steric bulk of its substituents, the metrical parameters of the cyclotriphosphane substructure were not perturbed compared to
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Synthesis of 3, depicting putative intermediates 5 and
previously reported P3 cycles. 17 Presumably, formation of 3 occurs first through 5, which loses anthracene to generate 1 that then rapidly trimerizes. Low temperature NMR experiments between –60 and 23 ◦ C did not lead to the observation of 5 or 1. To corroborate the intermediacy of 1, a thawing solution of Na[4] was mixed with a thawing solution of ClPA and NapS2 (peri-1,8-naphthalene disulfide, 1.0 equiv), 18 the latter serving as a trap for putative 1. Upon work-up, the vanadium iminophosphine 6 was isolated in 80% yield. The crystal structure of 6 displayed a V–N triple bond with a length of 1.6947(2) ˚ A, and a N–P single bond length of 1.74993(2) ˚ A(Figure 2). Importantly, formation of 6 was unlikely to involve 3, as no reactivity has been observed between 3 and NapS2 after 2 h at 23 ◦ C. An alternative associative mechanism involving reaction of NapS2 with 5 was similarly judged improbable upon finding that Me2 NPA reacted very slowly with NapS2 , forming less than 40% of NapS2 PNMe2 in 24 h at 23 ◦ C. S
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mechanism rather than through 1 in solution. However, such a mechanism is unlikely in the case of 3 transforming to 2 for steric reasons. The formation of oligomers such as 2 and 3 is highly reminiscent of phosphinidene reactivity, 6,19,20 leading us to t ¨ consider :P–N≡V(N[ Bu]Ar)3 as a possible resonance structure of putative 1 (see Figure 2 and SI). While the formation of diphosphenes or cyclic oligomers is a common reactivity pathway for phosphinidenes, 20,21 the transfer of a phosphinidene unit from a bare cyclo-triphosphane under such mild conditions is unprecedented. 22,23 In analogy to the predicted increased stability of ketimide substituted phosphinidenes, 24 the vanadium nitride substituent appears especially well-suited to stabilize low-valent phosphorus via electronic delocalization across the V-N-P π system. 14 Our computational studies corroborate this assertion, revealing a remarkably low P=P bond dissociation enthalpy of 41.2 kcal/mol for the model diphosphene [PNV(NMe2 )3 ]2 , sharply contrasting with the value for [PN(SiMe3 )2 ]2 of 80.3 kcal/mol (see SI).
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Scheme 2.
Synthesis of 6 in the presence of NapS2 .
This same product 6 was observed upon heating a benzene-d6 solution of 3 in the presence of 3 equiv. NapS2 to 55 ◦ C for 18 h, consistent with thermal transfer of 1 from 3. Integrations by 1 H NMR spectroscopy showed consumption of only one equivalent of NapS2 to generate 6. Accompanying the formation of 6, a new resonance centered at 191 ppm (∆ν1/2 450 Hz) was identified in the 51 V NMR spectrum along with a 31 P{1 H} NMR resonance characteristic of diphosphenes at 643 ppm (∆ν1/2 170 Hz). In the absence of the NapS2 trap, heating 3 at 70 ◦ C for 36 h resulted in complete conversion of 3 to diphosphene 2, isolated in 90% yield. The compound was unambiguously identified by Xray crystallography after crystallization from benzene. Once again, the short V–N length of 1.6933(1) ˚ A was consistent with a triple bond while the P–N bond length of 1.6530(1) ˚ A was suggestive of a single bond. However, the P–P bond length of 2.0787(7) ˚ A appeared significantly shorter than the P–P single bonds of 3, being in the expected range for a diphosphene P=P double bond. Together, these data suggest that 3 undergoes thermal [2+1]-retrocycloaddition to generate monomeric 1, prone to rapid dimerization. Admittedly, it is conceivable that generation of 6 from 3 and NapS2 at high temperatures occurs through an associative
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In an effort to transfer a monomeric unit from 3 to a substrate less likely to react through an associative mechanism, 3 was stirred in the presence of excess bis(trimethylsilyl)acetylene at 23 ◦ C in toluene (Scheme 4). After 48 h, a new product was observed at −104 ppm (∆ν1/2 320 Hz) by 51 V NMR spectroscopy. Warming the solution to 55 ◦ C drove the reaction to completion within 20 h; the target phosphirene 7 was isolated from the reaction mixture in 60% yield. Me3Si Me3SiC CSiMe3 55 °C, 20 h 60%
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Scheme 4. Preparation of phosphirene 7 and phosphirane 8 from thermolysis of 3.
Similar thermolysis of a benzene solution of 3 at 55 ◦ C in the presence of excess cis-4-octene led to the formation of 8, isolated in 70% yield. 1 H NMR spectroscopy and a single crystal X-ray diffraction study (Figure 3) confirmed formation of a single stereoisomer. Such stereospecific cycloaddition often implicates a singlet intermediate according to the Skell hypothesis 25 and is reminiscent of previously reported
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Figure 3. The molecular structure of 8 from a single crystal Xray diffraction study at 100 K, shown with 50% probability ellipsoids. Non-phosphiranyl hydrogen atoms and positional disorder are omitted for clarity. Selected interatomic distances [˚ A] and angles [◦ ]: V1-N1 1.6799(2), V1-N2 1.894(2), N1-P1 1.7164(2), P1-C4 1.878(2), P1-C5 1.879(3); V1-N1-P1 165.83(1), N1-P1-C4 103.52(1), N1-P1-C5 105.30(1), C4-P1-C5: 47.44(1).
phosphinidene reactivity. 6,26 Singlet reactivity of putative 1 was unexpected, as CASSCF–NEVPT2 quantum chemical calculations on the model complex PNV(NH2 )3 predicted a triplet ground state with a singlet-triplet gap of 12 kcal/mol (see SI). Stepwise addition of a triplet phosphinidene would have been expected to result in scrambling to form some amount of the trans isomer. 27 To further investigate the spin state of presumed monomer transfer, a benzene solution of 3 was heated at 55 ◦ C for 72 h in the presence of 10 equiv. vinylcyclopropane as a radical clock (Scheme 5). 28 Upon work-up, 9 was isolated as the major species. As evidenced by multinuclear and 2D NMR spectroscopy as well as X-ray crystallography, 9 was formed stereoselectively, most likely due to the bulky anilide ligand environment. More importantly, these data confirmed that 9 contained an intact cyclopropane ring, consistent with concerted singlet phosphinidene transfer (Figure 4). Vinylcyclopropane’s high ring opening rate constant of 1.3×108 s−1 makes a triplet intermediate unlikely, though a stepwise addition mechanism in which the rate of P-C bond formation is much faster than the rate of cyclopropane ring opening would also generate 9. 27 With a radical ringopening rate constant of 3.4×1011 s−1 , the 2,2-difluorocyclopropylcarbinyl radical is one the shortest lived calibrated radical clocks. 29 Repeating the transfer experiment with 1,1difluoro-2-vinylcyclopropane yielded two diastereoisomers of 10, as evidenced by multinuclear and 2D NMR spectroscopy, allowing reasonable exclusion of a triplet reactive intermediate. These data are consistent with fragmentation of 3 to singlet 1, which is rapidly intercepted before intersystem crossing to the more stable triplet state. 27 Such behavior is well known for carbenes. 30 A kinetic analysis on the transformation of 3 into 2 was prevented by the low solubility of 2, but the reaction between 3 and bis(trimethylsilyl)acetylene could be monitored
Figure 4. The solid-state structure of 9 shown with 50% probability thermal ellipsoids. Only the major part is depicted, and non-phosphiranyl hydrogen atoms are omitted for clarity. Selected interatomic distances [˚ A] and angles [◦ ]: V1-N1 1.675(2), V1-N2 1.894(2), N1-P1 1.732(2), P1-C1 1.842(4),P1-C2 1.882(3), C2-C3 1.497(5); V1-N1-P1 160.09(2), N1-P1-C1 103.82(2), C1P1-C2 47.35(2), C2-C1-P1 67.7(2), C3-C2-P1 116.4(3).
by NMR spectroscopy to investigate dissociative production of putative 1. The time evolution of the various implicated species can be rationalized according to equations 1–3. 3 2 + [1]
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In a reversible process, 3 presumably fragments via [2+1]retrocycloaddition, giving rise to the reactive intermediate 1, which can then either engage in [2+1]-cycloaddition with bis(trimethylsilyl)acetylene or dimerize to generate 2. Utilizing a large excess of bis(trimethylsilyl)acetylene, pseudo-firstorder conditions can be achieved such that ∂[7]/∂t = kobs [3], assuming that the forward direction of eq 1 is the rate limiting step. The decay of 3 was thus monitored by 51 V NMR spectroscopy at 66 ◦ C in the presence of excess alkyne, revealing the anticipated first order behavior in 3 in accordance with unimolecular decay (kobs = 1.34±0.14 s−1 ). Halving the concentration of alkyne did not produce a corresponding change in kobs , suggesting zeroth-order dependence. Together, these data indicate that an associative mechanism involving direct reaction of the alkyne with 3 is unlikely. Supporting the equilibrium depicted in eq 1, decay of 3 to 2 in the absence of a trap required 13 h at 66 ◦ C in THF, standing in sharp contrast with the 30 min required for complete decay of 3 in the presence of bis(trimethylsilyl)acetylene.
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References
Keq = 1.32±0.04 Me3Si
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Scheme 6. Thermolysis of 7 and 8 to generate 2 and substrate “hopping” between 7 and 8.
The isolobality of alkenes and diphosphenes draws a clear analogy between 3 and 8, leading one to question if 8 (or even 7) can serve as sources of 1. Heating a solution of 7 or 8 in benzene-d6 at 70 ◦ C for 18 h resulted in the formation of 2, releasing bis(trimethylsilyl)acetylene or cis-4-octene respectively, suggesting that both 7 and 8 can indeed serve as formal sources of 1. To test this hypothesis, a solution of 8 and excess bis(trimethylsilyl)acetylene in benzene-d6 was heated at 70 ◦ C for 18 h, resulting in 82% conversion to 7. In the same way, heating a solution of 7 in the presence of excess cis-4-octene resulted in 84% conversion to 8. In the presence of equimolar amounts of cis-4-octene and bis(trimethylsilyl)acetylene, it was possible to find that Keq = [8]/[7] = 1.32±0.04. It is noteworthy that no trans-4octene was detected throughout all the transfer experiments, consistent with singlet phosphinidene transfer. While phosphorus(III/V) redox cycles are well established, 31 reversible binding of unsaturated hydrocarbons through a phosphorus(I/III) cycle is highly reminiscent of transition-metal behavior. 32 Facile transfer of 1 to unactivated olefins complements the recent report of an isolable phosphinidene by Bertrand for which only binding to the strongly activated maleic anhydride was reported. 26 Phosphinidene “hopping” between olefins is more evocative of recent reports from Mathey 23 and Lammertsma 33 on metal-bound phosphinidene generation from phosphirane transition metal complexes. A molecular precursor has been utilized to prepare unique organometallic complexes featuring an unconventional PN linkage, providing an exciting first glimpse into the chemistry of complexed PN as a reactive intermediate. The facile transfer of a phosphinidenoid unit from 3, 7, and 8 suggests that the V(N[t Bu]Ar)3 platform could be further exploited in the quest for stabilizing reactive main group species. Current work is focused on expanding upon the reactivity of the interstellar molecule PN 34 as well as modifying the supporting anilide ligand framework with the goal of obtaining an isolable monomeric PN complex. Acknowledgement This material is based on research supported by the National Science Foundation under CHE1362118. M.A.C. acknowledges the NSERC for a postdoctoral fellowship. Dr. Bruce Adams is acknowledged for invaluable assistance with NMR experiments. Supporting Information Available: Experimental details, characterization data, X-ray crystallographic information, computational details, and tables of Cartesian coordinates are provided in the Supporting Information document. This material is available free of charge via the Internet at http://pubs.acs.org/.
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N BuAr ACS Paragon Plus Environment ArtBuN
B
ArtBuN
V
t
Ar BuN
C
NtBuAr
Me3SiSociety Journal of the American Chemical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
3
Me3SiC CSiMe3 55 °C, 20 h 60% –2
[1]
P
C3H7
cis-4-octene 55Paragon °C, 40Plus h Environment ACS 70%
H
NV(NtBuAr)3
7
Me3Si H
Page 10 of 15
P
C3H7
NV(NtBuAr)3
8
Page 11 ofJournal 15 of the American Chemical Society
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ACS Paragon Plus Environment
R
2 Journal of the American Chemical Society
R R
1 2 3 4 5 6 7 8 9 10
3
1
–2
[1]
55 °C, 72 h ACS Paragon Plus Environment
P
Page 12 of 15
NV(NtBuAr)3
9 (R = H) 10 (R = F)
Page 13 of 15 Journal of the American Chemical Society
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ACS Paragon Plus Environment
1:1Journal cis-4-octene/Me SiC CSiMe3 of the American3 Chemical Society 1 2 3 4 Si Me 3 5 6 7 8 9 10 11 Si Me 12 3 13 14 15 16 17
Page 14 of 15
Keq = 1.32±0.04 H
P
NV(NtBuAr)3
C3H7
P H
7
C3H7
NV(NtBuAr)3
8
70 °C, 48ACS h Paragon Plus Environment 70 °C, 18 h – cis-4-octene
2
– Me3SiC CSiMe3
Page 15 of 15
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Journal of the American Chemical Society
85x47mm (300 x 300 DPI)
ACS Paragon Plus Environment
