Communication pubs.acs.org/Organometallics
Synthesis and Properties of Naphtho[2,3‑e]‑1,2-azaphosphorine 2‑Oxides: PN-Anthracene Analogues Noah A. Takaesu,† Eisuke Ohta,†,§ Lev N. Zakharov,‡ Darren W. Johnson,*,† and Michael M. Haley*,† †
Department of Chemistry & Biochemistry and the Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253, United States ‡ CAMCOR, University of Oregon, Eugene, Oregon 97403-1433, United States S Supporting Information *
ABSTRACT: We report the expedient synthesis of a series of new “PN-anthracene” derivatives, the naphtho[2,3-e]-1,2-azaphosphorine-2-oxides, which incorporate a variety of 3-aryl substituents. These compounds represent a new class of π-extended PN-heterocycles, featuring strong UV absorption, emission in the visible range, and Stokes shifts up to 10000 cm−1. These molecules show alignment of dipoles, strong dimerization through N−H and PO hydrogen bond donors/acceptors, and efficient aromatic stacking in the solid state. phosphonamidates 7a−d, the first examples of anthracene-like PN molecules. Starting from amine 8,8 Sonogashira cross-coupling with excess trimethylsilylacetylene (TMSA) afforded silane 9 in very good yield (Scheme 1). Base-induced desilation followed by a second cross-coupling gave the requisite tolanes 10a−d. Finally, cyclization with P(OPh)3 overnight at 100 °C and then repeated recrystallization of the crude material furnished phosphonamidates 7a−d in modest to moderate yields as bright yellow-orange powders. Single crystals of 7c suitable for X-ray diffraction were grown via slow evaporation of a CDCl3 solution at room temperature. The molecular structure and packing in the solid state are shown in Figure 2. The P(1)−N(1) bond length (1.6346 Å) is typical of an amidate-type structure, and C(1)−C(2) (1.350 Å) is best described as an isolated double bond. The remaining C− C bond lengths of the tricyclic core are consistent with those of fused naphthalene derivatives. The longer P(1)−N(1) and P(1)−C(1) (1.7816 Å) bond lengths do lead to some distortion of the six-membered heterocycle, as reflected by the large C(1)−C(2)−C(3) and C(4)−N(1)−P(1) bond angles (127.53 and 127.20°, respectively) as well as by the deviation of the P atom 0.250 Å from the plane defined by the other 13 atoms in the tricyclic core (rms deviation from planarity 0.0308 Å). Excluding the P atom, the plane defined by the other 13 atoms exhibits a dihedral angle of 26.3° with the pendant trifluoromethylphenyl group, suggesting good electronic communication between the two π systems. Phosphonamidate 7c crystallizes as a dimer between the two enantiomers, forming a complementary association between the N−H donor and PO acceptor groups (Figure 2) with a short
P
N-heterocycles such as azaphosphinines/azaphosphorines have been investigated in the literature for many decades.1 Two of the earliest examples are phosphinamidate 1 and phosphonamidate 2 (Figure 1), reported nearly simultaneously
Figure 1. Known multiring structures that contain six-membered PNheterocycles.
in 1960 by Dewar2 and Campbell.3 More recent studies have focused on high-yield, metal-mediated routes to phosphonamidates such as 3.4 A series of closely related phosphinamides (e.g., 4) have also become a hot topic in the literature recently as a route to chiral phosphines.5 Despite the current resurgence in interest,6 expanded multiring structures that contain such sixmembered PN-heterocyclic motifs are rather rare, aside from the few phenanthrene-like molecules shown above. We recently described a metal-free synthesis of sixmembered PN-heterocycles using P(OPh)3 starting from readily available o-ethynylaniline derivatives to give either the imidate (e.g., 5) or amidate form (e.g., 6), depending upon the exclusion or inclusion of water, respectively, in moderate to very good yield.7 Given the dearth of ring-expanded, conjugated PN-heterocycles, we sought to explore the structural and optoelectronic properties of such systems. Here we report our initial foray into the preparation and characterization of © XXXX American Chemical Society
Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: April 12, 2017
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DOI: 10.1021/acs.organomet.7b00281 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
intermolecular distance (N···O 2.767 Å) and nearly linear N− H···O angle (169.1°). The molecules are arranged in a centrosymmetric pattern, canceling out molecular dipoles. This orientation also features close aromatic stacking interactions between neighboring conjugated cores, with a distance between the 13-atom average planes of 3.432 Å. Figure 3 shows the electronic absorption and emission spectra of 7a−d; the data are compiled in Table 1. Interestingly,
Scheme 1. Facile Synthesis of PN-Heterocycles 7
Figure 3. Electronic absorption (solid line) and emission (dashed line) spectra of PN-heterocycles 7a−d.
all four molecules absorb over a rather narrow 12 nm range for λmaxabs, values that are seemingly insensitive to the nature of the R substituent on 7. On the other hand, the fluorescence spectra are significantly influenced by the R substituent, with the electron-donating MeO group exhibiting a λmaxem value at 464 nm and the electron-withdrawing CN group at 506 nm, with Stokes shifts of 8000 and 10000 cm−1, respectively. The PN functionality is important, as the amine precursors 10 are only weakly fluorescent (QY < 1%) and are considerably blue shifted (∼50 nm) in comparison to the final PN-heterocycles. These enhanced electronic properties of 7 can be attributed in part to inclusion of the extra fused benzene, as the λmaxem and Stokes shifts of the analogous derivatives of 6 are in the 410−430 nm and 5000−6000 cm−1 ranges, respectively.7 Although it is possible to envisage an excited state proton transfer from the N−H to PO group, solvent effects on the fluorescence properties are minimalheterocycles 7 exhibit weak, positive solvatochromism (ca. 15−20 nm) on switching from CHCl3 to DMF. Nonetheless, the large Stokes shifts for 7a−d hint at the possibility of use of suitably derivatized versions of this scaffold for imaging and sensing applications,9 given the near absence of overlap between excitation and emission spectra of the molecules (Figure 3 and Figure S1 in the Supporting Information). In summary, we report the expedient synthesis of a series of new “PN-anthracenes”: the naphtho[2,3-e]-1,2-azaphosphorine 2-oxides featuring a variety of 3-aryl substituents. These compounds represent a new class of extended PN-heterocycles, featuring strong UV absorption, emission in the visible range, and Stokes shifts up to 10000 cm−1. The potential to tune these emission properties out into the near-IR in a facilely synthesized, compact fluorescent scaffold suggests longer-term applications in molecular probes and imaging. Furthermore, the alignment of dipoles, strong dimerization tendency through N− H and PO hydrogen bond donors/acceptors, and efficient aromatic stacking in the solid state suggest that this class of
Figure 2. Molecular structure (top) and packing (bottom) of PNheterocycle 7c. Selected bond lengths (Å) and bond angles (deg): P(1)−O(1) 1.6066(13), P(1)−O(2) 1.4745(13), P(1)−C(1) 1.7816(18), P(1)−N(1) 1.6346(15), C(1)−C(2) 1.350(2), C(2)− C(3) 1.450(2), C(3)−C(4) 1.424(2), C(4)−N(1) 1.396(2); N(1)− P(1)−C(1) 103.93(8), P(1)−C(1)−C(2) 118.12(13), C(1)−C(2)− C(3) 127.53(16), C(2)−C(3)−C(4) 121.07(16), C(3)−C(4)−N(1) 119.15(15), C(4)−N(1)−P(1) 127.20(12).
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DOI: 10.1021/acs.organomet.7b00281 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Table 1. Optical Data for PN-Heterocycles 7a
a
compd
λmaxabs (nm)
ε (M−1 cm−1)
λmaxem (nm)
Stokes shift (nm/cm−1)
QY (%)b
7a 7b 7c 7d
339 331 327 336
27210 26900 21120 30800
464 469 492 506
125/7917 138/8890 165/10256 170/9999
7.1 9.0 1.9 4.0
Spectra were obtained as 1−5 mM solutions in CHCl3. bDetermined using quinine as standard. (2) Dewar, M. J. S.; Kubba, V. P. J. Am. Chem. Soc. 1960, 82, 5685− 5688. (3) (a) Campbell, I. G. M.; Way, J. K. J. Chem. Soc. 1960, 0, 5034− 5041. (b) Campbell, I. G. M.; Way, J. K. J. Chem. Soc. 1961, 2133− 2141. (4) Inter alia: (a) Tang, W.; Ding, Y.-X. J. Org. Chem. 2006, 71, 8489−8492. (b) Tang, W.; Ding, Y.; Ding, Y.-X. Tetrahedron 2008, 64, 10507−10511. (c) Yan, J.; Li, Q.; Boutin, J. A.; Renard, M. P.; Ding, Y.; Hao, X.; Zhao, W.; Wang, M. Acta Pharmacol. Sin. 2008, 29, 752− 758. (d) Park, S.; Seo, B.; Shin, S.; Son, J.-Y.; Lee, P. H. Chem. Commun. 2013, 49, 8671−8673. (e) Zhao, D.; Nimphius, C.; Lindale, M.; Glorius, F. Org. Lett. 2013, 15, 4504−4507. (5) Inter alia: (a) Lin, Z.-Q.; Wang, W.-Z.; Yan, S.-B.; Duan, W.-L. Angew. Chem., Int. Ed. 2015, 54, 6265−6269. (b) Liu, L.; Zhang, A.-A.; Wang, Y.; Zhang, F.; Zuo, Z.; Zhao, W.-X.; Feng, C.-L.; Ma, W. Org. Lett. 2015, 17, 2046−2049. (c) Sun, Y.; Cramer, N. Angew. Chem., Int. Ed. 2017, 56, 364−367. (d) Ma, Y.-N.; Zhang, X.; Yang, S.-D. Chem. Eur. J. 2017, 23, 3007−3011. (6) See also: (a) Stolar, M.; Baumgartner, T. Phosphorus-Containing Polycyclic Heteroarenes. In Polycyclic Arenes and Heteroarenes: Synthesis, Properties, and Applications; Miao, Q., Ed.; Wiley-VCH: Weinheim, Germany, 2015; pp 309−329. (b) Hindenberg, P.; Romero-Nieto, C. Synlett 2016, 27, 2293−2300. (c) Yoshikai, N.; Santra, M.; Wu, B. Organometallics 2017, DOI: 10.1021/acs.organomet.7b00244. (7) Vonnegut, C. L.; Shonkwiler, A. M.; Khalifa, M. K.; Zakharov, L. N.; Johnson, D. W.; Haley, M. M. Angew. Chem., Int. Ed. 2015, 54, 13318−13322. (8) Ishibashi, J. S. A.; Marshall, J. L.; Maziere, A.; Lovinger, G. J.; Li, B.; Zakharov, L. N.; Dargelos, A.; Graciaa, A.; Chrostowska, A.; Liu, S.Y. J. Am. Chem. Soc. 2014, 136, 15414−15421. (9) Araneda, J. F.; Piers, W. E.; Heyne, B.; Parvez, M.; McDonald, R. Angew. Chem., Int. Ed. 2011, 50, 12214−12217.
molecules warrants investigations in organic electronics applications as well. Future studies will target further extension of the linear chromophore as well as the effect of angular fusion on molecular properties.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00281. Experimental procedures, characterization data, crystallographic data for 7c, and spectra (PDF) Accession Codes
CCDC 1550413 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for D.W.J.:
[email protected]. *E-mail for M.M.H.:
[email protected]. ORCID
Darren W. Johnson: 0000-0001-5967-5115 Michael M. Haley: 0000-0002-7027-4141 Present Address §
E.O.: Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-1607214) for support of the research, as well as for support in the form of an instrumentation grant (CHE-1427987). HRMS were obtained at the Biomolecular Mass Spectrometry Core of the Environmental Health Sciences Core Center at Oregon State University (NIH P30ES000210). We thank Dan Seidenkranz for his assistance in the HPLC analysis of 7a−d.
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REFERENCES
(1) (a) Mathey, F.; le Floch, P. Product Class 13:1-λ3-Phosphinines. In Science of Synthesis; Black, D. StC., Ed.; Georg Thieme: Stuttgart, Germany, 2005; Vol. 15, pp 1097−1155. (b) Streubel, R. Product Class 14:1-λ5-Phosphinines. In Science of Synthesis; Black, D. StC., Ed.; Georg Thieme: Stuttgart, Germany, 2005; Vol. 15, pp 1157−1180. (c) Bergsträsser, U. Product Class 15: Benzo-fused and Other Annulated Phosphinines. In Science of Synthesis; Black, D. StC., Ed.; Georg Thieme: Stuttgart, Germany, 2005; Vol. 15, pp 1181−1190. C
DOI: 10.1021/acs.organomet.7b00281 Organometallics XXXX, XXX, XXX−XXX
