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Cite this: Dalton Trans., 2015, 44, 54 Received 11th October 2014, Accepted 4th November 2014 DOI: 10.1039/c4dt03136a www.rsc.org/dalton
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Cleavage of an aryl carbon–nitrogen bond of a phosphazido iron(II) complex promoted by hydride metathesis† Takahiko Ogawa,a Tatsuya Suzuki,a Nicholas M. Hein,b Fraser S. Pickb and Michael D. Fryzuk*b
Upon reaction with KBEt3H, the pseudo tetrahedral Fe(II) complex 5 with a bulky enamido-phosphazide ligand set undergoes elimination of N2 and 1,3-Me2C6H4 to generate the dinuclear Fe(II) derivative 6 with bridging phosphinimido units. When the reaction is performed using KBEt3D, no deuterium is incorporated into the eliminated 1,3Me2C6H4; all of the deuterium ends up as D2. When the reaction is performed in THF-d8, only 2-d-1,3-Me2C6H3D was detected by GCMS. These studies are consistent with a radical mechanism.
Besides being an important method for the preparation of amines from azides, the Staudinger reaction1 also has garnered increased attention in ligand design2 as a method to functionalize a phosphine donor to an iminophosphorane (phosphazene).3,4 Studies have shown that the intermediate phosphazide species can be intercepted in certain cases,5 usually by the use of sterically hindered phosphines and/or azides.3,5–8 Other ways to access this intermediate have involved appropriate choice of substituents to electronically stabilize the phosphazide, coordination to metal ions7,9 or Lewis acids,10,11 the presence of hydrogen bonding,10,12 and the use of cyclic structures that impede the formation of the cis form.13 In this paper we report a combination of sterics and intramolecular hydrogen bonding as a method to stabilize the phosphazide intermediate, which has allowed us to further generate coordination complexes with potassium and iron(II). Furthermore we also include our studies on an unusual reaction with hydride that results in the cleavage of the carbon– nitrogen bond of the aryl azide. Upon reaction of the readily available cyclopentylidene-based phosphine-imine 114 with xylylazide (2,6-Me2C6H3-N3), we were able to isolate the phosphazide 2, which exists exclusively in the
enamine-phosphazide form as shown in Scheme 1; most diagnostic for the enamine tautomer is the downfield singlet in the 1 H NMR spectra at δ 10.9 assigned to the N–H moiety. Subsequent heating overnight at 80 °C results in the formation of the iminophosphorane 3 in excellent yield (see ESI†). The X-ray crystal structure shown in Fig. 1 confirms the presence of the triaza unit between the phosphorus and 2,6dimethylphenyl moieties in 2. The phosphazide is in the s-trans geometry, no doubt due to the bulk of the aryl unit and the isopropyl substituents at phosphorus. In addition, the presence of a hydrogen bond between the enamine N–H and N2 of the triaza unit likely adds to the stability of the phosphazide; the distance between H1 and N2 is 2.046(16) Å. Other examples10,12 of hydrogen bonding to stabilize the phosphazide unit have involved the distal nitrogen atom (to the phosphorus), which makes the observed hydrogen bond here different. Also consistent with the enamine-phosphazide formalism are the short C1–C2 (1.3668(13) Å), P1–N2 (1.6393(9) Å), and N3–N4 (1.2665(11) Å) bond lengths, which are best described as double bonds.
ð1Þ
a
Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada, V6 T 1Z1. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, X-ray data collection and refinement procedures for 2, 4, 5, 6; characterization by NMR, Mössbauer; GCMS results for deuterium labeling experiments. CCDC 1028480–1028483. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03136a b
54 | Dalton Trans., 2015, 44, 54–57
Scheme 1
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Fig. 1 ORTEP drawing of the solid-state molecular structure of 2 (ellipsoids at 50% probability level). All hydrogen atoms except for H1 have been omitted for clarity; H1 was located from the difference map. Selected bond lengths (Å) and angles (°): N1–C1: 1.3657(12), C1–C2: 1.3668(13), C2–P1: 1.7540(10), P1–N2: 1.6393(9), N2–N3: 1.3583(11), N3–N4: 1.2665(11), H1⋯N2: 2.046(16), C1–N1–C6: 122.10(8), N1–C1– C2: 128.07(9), C2–P1–N2: 105.37(4), P1–N2–N3: 110.29(6), N2–N3–N4: 112.92(8), N3–N4–C24: 111.18(8).
Enamine-phosphazide 2 can be deprotonated by KH in THF at room temperature to form the potassium derivative 4 (eqn (1)), which displays a singlet resonance at δ 49.0 in the 31P NMR spectrum, only slightly upfield-shifted from the starting phosphazide at δ 50.9. Analysis of 4 by X-ray diffraction revealed that the K+ ion is bound in a κ3 fashion to the triaza portion of the molecule (see ESI†); the X-ray structure also indicates that this material is polymeric in the solid state. Because the potassium salt is soluble in aromatic hydrocarbons, it is likely that the intermolecular interaction is not present in solution. We have initiated a study of the coordination chemistry of this deprotonated-enamine-phosphazide aimed at examining the analogy to NacNac type ligands. Reaction of 4 with FeBr2·2THF results in the formation of corresponding adduct 5 as shown in Scheme 2. What is evident from the solid-state structure is that the enamido-phosphazide form that acts as a bidentate ligand in binding to Fe(II) in 5, with only the α-nitrogen (N2) of the triaza unit bound to Fe (Fig. 2). The geometry around the Fe(II) center is tetrahedral with both Fe–N bond distances nearly identical at 2.04 Å. The Fe(II) complex is paramagnetic with μeff = 4.8 BM (4 unpaired electrons) based on the Evans method. Stabilization of phosphazides by coordination to metal complexes has been reported previously.7,9 There are examples that illustrate the different bonding modes possible with this triaza unit, which include
Scheme 2
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Fig. 2 ORTEP drawing of the solid-state molecular structure of 5 (ellipsoids at 50% probability level). All hydrogen atoms have been omitted for clarity. Selected bond length (Å) and angles (°): Fe1–N1: 2.032(7), Fe1–N2: 2.039(7), Fe1–Br1: 2.417(2), Fe1–O1: 2.122(5), N1–C1: 1.348(10), C1–C2: 1.398(12), C2–P1: 1.726(9), P1–N2: 1.660(7), N2–N3: 1.354(9), N3–N4: 1.285(9), N1–Fe1–N2: 89.7(3), N1–Fe1–Br1: 121.5(2), N2–Fe1–Br1: 115.2(2), C1–N1–C6: 116.8(7), N1–C1–C2: 128.3(8), C1–C2–P1: 128.1(6), C2–P1–N2: 106.1(4), P1–N2–N3: 114.3(6), N2–N3–N4: 108.9(7).
bidentate and monodentate interactions for the nitrogen atoms. The combination of a proximal enamido and a phosphazide unit generates the six-membered chelate ring geometry, which is very reminiscent of NacNac type ligands. Reaction with hydride reagent KBEt3H was examined to try and access hydride dimers similar to ([NacNac]MH)2 (M = Fe, Co), which have proven to be important in small molecule activation.15 The addition of KBEt3H in THF to 5 results in the low yield formation of a new paramagnetic iron complex, the structure of which is shown in Fig. 3. As indicated in Scheme 2, the product of the reaction is a dinuclear complex that contains bridging phosphinimido units; the Fe–Fe distance is 2.4995(10) Å. The bond lengths around the core of the diiron unit are not unusual as shown in
Fig. 3 ORTEP drawing of the solid-state molecular structure of 6 (ellipsoids at 50% probability level). All hydrogen atoms have been omitted for clarity. Selected bond length (Å) and angles (°): Fe1–N1: 1.9303(13), Fe1–N2: 1.9247(13), Fe1–N2’: 1.8678(14), Fe1–Fe1’: 2.4996(9), N1–C1: 1.3578(17), C1–C2: 1.3809(17), C2–P1: 1.7654(14), P1–N2: 1.5847(13), N1–Fe1–N2: 107.11(5), Fe1–N2–Fe1’: 82.44(5), N2–Fe1–N2’: 97.55(5), C2–P1–N2: 113.94(7).
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phosphinimido unit without the intermediacy of a phosphinimine. Deuterium labeling experiments are consistent with a radical process. While the phosphazide moiety can act as a ligand, it is clear that it may also be a precursor to phosphinimido units. In the Staudinger process, it is the nitrogen that is attached to the azide unit that is retained upon thermolysis. In contrast, this hydride metathesis process results in the distal nitrogen to the organoazide being transferred to the phosphine.
Acknowledgements We thank NSERC of Canada for a Discovery grant to M. D. F. We also thank the Japan Society for the Promotion of Science for the grant “Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation” to T. O. and T. S. Scheme 3
Notes and references the caption in Fig. 3; these bond lengths are consistent with some delocalization around the N1–C13–C17–P1 unit. The measured magnetic moment for dimer 6 is 3.1 BM, which is consistent with two high spin Fe(II) centers antiferromagnetically coupled. The key transformation that has occurred is that the phosphazide unit has been converted into a phosphinimido unit via loss of N2 and xylene (1,3-Me2C6H4). When the reaction was performed with the corresponding deuteride, KBEt3D, no deuterium was detected in the xylene. However, when the reaction was performed in deuterated THF (THF-d8), 2-d1-xylene was generated as confirmed by GC-MS and 13C{1H} NMR spectroscopy (see ESI†). On the basis of these two experiments, we can rule out direct reaction of the hydride at the C–N bond of the coordinated phosphazide unit, or even intramolecular H-transfer from the Fe center to the ipso carbon of the arylazido unit, and instead invoke a radical process that generates an aryl radical at the 2-position of xylene, which subsequently abstracts a hydrogen or a deuterium from the solvent. One possible process is shown in Scheme 3, which involves the formation of the putative hydride 7, followed by elimination of H•, detected as H2 (or D2 for the reaction with KBEt3D),16 and a one-electron cascade that starts with a Fe(I) center and results in the generation of N2 and the xylyl radical;17 subsequent H or D abstraction from the protio or deutero solvent produces xylene or 2-d1-xylene, respectively. Once formed, the sterically unencumbered imine-phosphinimido iron(II) complex dimerizes to form 6.
Conclusions The surprising aspect of this work is not the isolation of a thermally stable phosphazide moiety that can also act as a ligand and remain intact upon coordination to iron(II); rather it is the hydride-induced conversion of the phosphazide unit into a
56 | Dalton Trans., 2015, 44, 54–57
1 (a) Y. G. Gololobov and L. F. Kasukhin, Tetrahedron, 1992, 48, 1353; (b) Y. G. Gololobov, I. N. Zhmurova and L. F. Kasukhin, Tetrahedron, 1981, 37, 437; (c) E. F. V. Scriven and K. Turnbull, Chem. Rev., 1988, 88, 297. 2 (a) T. Cantat, N. Mézailles, A. Auffrant and P. Le Floch, Dalton Trans., 2008, 1957; (b) O. J. Cooper, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Dalton Trans., 2010, 39, 5074; (c) D. Li, S. Li, D. Cui, X. Zhang and A. A. Trifonov, Dalton Trans., 2011, 40, 2151; (d) G. C. Welch, W. E. Piers, M. Parvez and R. McDonald, Organometallics, 2004, 23, 1811; (e) C. M. Ong, P. McKarns and D. W. Stephan, Organometallics, 1999, 18, 4197; (f ) R. P. Kamalesh Babu, R. McDonald and R. G. Cavell, Organometallics, 2000, 19, 3462; (g) A. J. Wooles, O. J. Cooper, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Organometallics, 2010, 29, 2315; (h) M. T. Gamer, P. W. Roesky, I. Palard, M. Le Hellaye and S. M. Guillaume, Organometallics, 2007, 26, 651; (i) M. Wiecko and P. W. Roesky, Organometallics, 2009, 28, 1266; ( j) B. Liu, D. Cui, J. Ma, X. Chen and X. Jing, Chem. – Eur. J., 2007, 13, 834; (k) B. Liu, X. Liu, D. Cui and L. Liu, Organometallics, 2009, 28, 1453; (l) R. Cariou, T. W. Graham, F. Dahcheh and D. W. Stephan, Dalton Trans., 2011, 40, 5419; (m) A. J. Wooles, M. Gregson, O. J. Cooper, A. Middleton-Gear, D. P. Mills, W. Lewis, A. J. Blake and S. T. Liddle, Organometallics, 2011, 30, 5314; (n) G. Ma, M. J. Ferguson, R. McDonald and R. G. Cavell, Inorg. Chem., 2011, 50, 6500; (o) T.-P.-A. Cao, A. Buchard, X. F. Le Goff, A. Auffrant and C. K. Williams, Inorg. Chem., 2012, 51, 2157. 3 M. W. P. Bebbington and D. Bourissou, Coord. Chem. Rev., 2009, 253, 1248. 4 (a) M. Alajarin, C. Conesa and H. S. Rzepa, J. Chem. Soc., Perkin Trans. 2, 1999, 1811; (b) G. M. Fang, C. Wang, J. Shi and Q. X. Guo, Acta Chim. Sin., 2009, 67, 2335; (c) W. Q. Tian
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7 8
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and Y. A. Wang, J. Org. Chem., 2004, 69, 4299; (d) W. Q. Tian and Y. A. Wang, J. Chem. Theory Comput., 2005, 1, 353; (e) C. Widauer, H.-J. Grützmacher, I. Shevchenko and V. Gramlich, Eur. J. Inorg. Chem., 1999, 1659. R. D. Kennedy, Chem. Commun., 2010, 46, 4782. G. C. Fortman, B. Captain and C. D. Hoff, Inorg. Chem., 2009, 48, 1808. L. LePichon and D. W. Stephan, Inorg. Chem., 2001, 40, 3827. P. Molina, C. LopezLeonardo, J. LlamasBotia, C. FocesFoces and C. FernandezCastano, Tetrahedron, 1996, 52, 9629. (a) K. Bieger, G. Bouhadir, R. Réau, F. Dahan and G. Bertrand, J. Am. Chem. Soc., 1996, 118, 1038; (b) E. M. Broderick, P. S. Thuy-Boun, N. Guo, C. S. Vogel, J. Sutter, J. T. Miller, K. Meyer and P. Diaconescu, Inorg. Chem., 2011, 50, 2870; (c) V. Cadierno, M. Zablocka, B. Donnadieu, A. Igau, J.-P. Majoral and A. Skowronska, Chem. – Eur. J., 2000, 6, 345; (d) G. L. Hillhouse, G. V. Goeden and B. L. Haymore, Inorg. Chem., 1982, 21, 2064. M. W. P. Bebbington, S. Bontemps, G. Bouhadir and D. Bourissou, Angew. Chem., Int. Ed., 2007, 46, 3333. A. Stute, L. Heletta, R. Frohlich, C. G. Daniliuc, G. Kehr and G. Erker, Chem. Commun., 2012, 48, 11739. C. G. Chidester, J. Szmuszkovicz, D. J. Duchamp, L. G. Laurian and J. P. Freeman, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1988, 1080.
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13 (a) M. Alajarín, P. Molina, A. López-Lázaro and C. FocesFoces, Angew. Chem., Int. Ed., 1997, 36, 67; (b) M. Alajarin, A. Lopez-Lazaro, A. Vidal and J. Berna, Chem. – Eur. J., 1998, 4, 2558; (c) M. Alajarin, C. Lopez-Leonardo and J. Berna, Tetrahedron, 2006, 62, 6190; (d) M. Alajarin, C. Lopez-Leonardo, J. Berna and J. W. Steed, Tetrahedron, 2007, 63, 2078. 14 (a) Z. Guan and J. W. Marshall, Organometallics, 2002, 21, 3580; (b) W. Keim, S. Killat, C. F. Nobile, G. P. Suranna, U. Englert, R. Wamg, S. Mecking and D. L. Schröder, J. Organomet. Chem., 2002, 662, 150; (c) T. Zhu, T. C. Wambach and M. D. Fryzuk, Inorg. Chem., 2011, 50, 11212. 15 (a) Y. Yu, A. R. Sadique, J. M. Smith, T. R. Dugan, R. E. Cowley, W. W. Brennessel, C. J. Flaschenriem, E. Bill, T. R. Cundari and P. L. Holland, J. Am. Chem. Soc., 2008, 130, 6624; (b) K. Ding, W. W. Brennessel and P. L. Holland, J. Am. Chem. Soc., 2009, 131, 10804; (c) J. Ballmann, R. F. Munhá and M. D. Fryzuk, Chem. Commun., 2010, 46, 1013. 16 Although 7 is shown (Scheme 3) as mononuclear, a dinuclear hydride bridged complex might facilitate H2 formation by combination of 2 H•. 17 Alternatively, the ArNN• radical may eliminate first, then go on to form Ar• and N2; see: C. E. LaPlaza, A. L. Odom, W. M. Davis and C. C. Cummins, J. Am. Chem. Soc., 1995, 117, 4999.
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Cite this: Dalton Trans., 2015, 44, 54 Received 11th October 2014, Accepted 4th November 2014 DOI: 10.1039/c4dt03136a www.rsc.org/dalton
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Cleavage of an aryl carbon–nitrogen bond of a phosphazido iron(II) complex promoted by hydride metathesis† Takahiko Ogawa,a Tatsuya Suzuki,a Nicholas M. Hein,b Fraser S. Pickb and Michael D. Fryzuk*b
Upon reaction with KBEt3H, the pseudo tetrahedral Fe(II) complex 5 with a bulky enamido-phosphazide ligand set undergoes elimination of N2 and 1,3-Me2C6H4 to generate the dinuclear Fe(II) derivative 6 with bridging phosphinimido units. When the reaction is performed using KBEt3D, no deuterium is incorporated into the eliminated 1,3Me2C6H4; all of the deuterium ends up as D2. When the reaction is performed in THF-d8, only 2-d-1,3-Me2C6H3D was detected by GCMS. These studies are consistent with a radical mechanism.
Besides being an important method for the preparation of amines from azides, the Staudinger reaction1 also has garnered increased attention in ligand design2 as a method to functionalize a phosphine donor to an iminophosphorane (phosphazene).3,4 Studies have shown that the intermediate phosphazide species can be intercepted in certain cases,5 usually by the use of sterically hindered phosphines and/or azides.3,5–8 Other ways to access this intermediate have involved appropriate choice of substituents to electronically stabilize the phosphazide, coordination to metal ions7,9 or Lewis acids,10,11 the presence of hydrogen bonding,10,12 and the use of cyclic structures that impede the formation of the cis form.13 In this paper we report a combination of sterics and intramolecular hydrogen bonding as a method to stabilize the phosphazide intermediate, which has allowed us to further generate coordination complexes with potassium and iron(II). Furthermore we also include our studies on an unusual reaction with hydride that results in the cleavage of the carbon– nitrogen bond of the aryl azide. Upon reaction of the readily available cyclopentylidene-based phosphine-imine 114 with xylylazide (2,6-Me2C6H3-N3), we were able to isolate the phosphazide 2, which exists exclusively in the
enamine-phosphazide form as shown in Scheme 1; most diagnostic for the enamine tautomer is the downfield singlet in the 1 H NMR spectra at δ 10.9 assigned to the N–H moiety. Subsequent heating overnight at 80 °C results in the formation of the iminophosphorane 3 in excellent yield (see ESI†). The X-ray crystal structure shown in Fig. 1 confirms the presence of the triaza unit between the phosphorus and 2,6dimethylphenyl moieties in 2. The phosphazide is in the s-trans geometry, no doubt due to the bulk of the aryl unit and the isopropyl substituents at phosphorus. In addition, the presence of a hydrogen bond between the enamine N–H and N2 of the triaza unit likely adds to the stability of the phosphazide; the distance between H1 and N2 is 2.046(16) Å. Other examples10,12 of hydrogen bonding to stabilize the phosphazide unit have involved the distal nitrogen atom (to the phosphorus), which makes the observed hydrogen bond here different. Also consistent with the enamine-phosphazide formalism are the short C1–C2 (1.3668(13) Å), P1–N2 (1.6393(9) Å), and N3–N4 (1.2665(11) Å) bond lengths, which are best described as double bonds.
ð1Þ
a
Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada, V6 T 1Z1. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, X-ray data collection and refinement procedures for 2, 4, 5, 6; characterization by NMR, Mössbauer; GCMS results for deuterium labeling experiments. CCDC 1028480–1028483. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03136a b
54 | Dalton Trans., 2015, 44, 54–57
Scheme 1
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Fig. 1 ORTEP drawing of the solid-state molecular structure of 2 (ellipsoids at 50% probability level). All hydrogen atoms except for H1 have been omitted for clarity; H1 was located from the difference map. Selected bond lengths (Å) and angles (°): N1–C1: 1.3657(12), C1–C2: 1.3668(13), C2–P1: 1.7540(10), P1–N2: 1.6393(9), N2–N3: 1.3583(11), N3–N4: 1.2665(11), H1⋯N2: 2.046(16), C1–N1–C6: 122.10(8), N1–C1– C2: 128.07(9), C2–P1–N2: 105.37(4), P1–N2–N3: 110.29(6), N2–N3–N4: 112.92(8), N3–N4–C24: 111.18(8).
Enamine-phosphazide 2 can be deprotonated by KH in THF at room temperature to form the potassium derivative 4 (eqn (1)), which displays a singlet resonance at δ 49.0 in the 31P NMR spectrum, only slightly upfield-shifted from the starting phosphazide at δ 50.9. Analysis of 4 by X-ray diffraction revealed that the K+ ion is bound in a κ3 fashion to the triaza portion of the molecule (see ESI†); the X-ray structure also indicates that this material is polymeric in the solid state. Because the potassium salt is soluble in aromatic hydrocarbons, it is likely that the intermolecular interaction is not present in solution. We have initiated a study of the coordination chemistry of this deprotonated-enamine-phosphazide aimed at examining the analogy to NacNac type ligands. Reaction of 4 with FeBr2·2THF results in the formation of corresponding adduct 5 as shown in Scheme 2. What is evident from the solid-state structure is that the enamido-phosphazide form that acts as a bidentate ligand in binding to Fe(II) in 5, with only the α-nitrogen (N2) of the triaza unit bound to Fe (Fig. 2). The geometry around the Fe(II) center is tetrahedral with both Fe–N bond distances nearly identical at 2.04 Å. The Fe(II) complex is paramagnetic with μeff = 4.8 BM (4 unpaired electrons) based on the Evans method. Stabilization of phosphazides by coordination to metal complexes has been reported previously.7,9 There are examples that illustrate the different bonding modes possible with this triaza unit, which include
Scheme 2
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Fig. 2 ORTEP drawing of the solid-state molecular structure of 5 (ellipsoids at 50% probability level). All hydrogen atoms have been omitted for clarity. Selected bond length (Å) and angles (°): Fe1–N1: 2.032(7), Fe1–N2: 2.039(7), Fe1–Br1: 2.417(2), Fe1–O1: 2.122(5), N1–C1: 1.348(10), C1–C2: 1.398(12), C2–P1: 1.726(9), P1–N2: 1.660(7), N2–N3: 1.354(9), N3–N4: 1.285(9), N1–Fe1–N2: 89.7(3), N1–Fe1–Br1: 121.5(2), N2–Fe1–Br1: 115.2(2), C1–N1–C6: 116.8(7), N1–C1–C2: 128.3(8), C1–C2–P1: 128.1(6), C2–P1–N2: 106.1(4), P1–N2–N3: 114.3(6), N2–N3–N4: 108.9(7).
bidentate and monodentate interactions for the nitrogen atoms. The combination of a proximal enamido and a phosphazide unit generates the six-membered chelate ring geometry, which is very reminiscent of NacNac type ligands. Reaction with hydride reagent KBEt3H was examined to try and access hydride dimers similar to ([NacNac]MH)2 (M = Fe, Co), which have proven to be important in small molecule activation.15 The addition of KBEt3H in THF to 5 results in the low yield formation of a new paramagnetic iron complex, the structure of which is shown in Fig. 3. As indicated in Scheme 2, the product of the reaction is a dinuclear complex that contains bridging phosphinimido units; the Fe–Fe distance is 2.4995(10) Å. The bond lengths around the core of the diiron unit are not unusual as shown in
Fig. 3 ORTEP drawing of the solid-state molecular structure of 6 (ellipsoids at 50% probability level). All hydrogen atoms have been omitted for clarity. Selected bond length (Å) and angles (°): Fe1–N1: 1.9303(13), Fe1–N2: 1.9247(13), Fe1–N2’: 1.8678(14), Fe1–Fe1’: 2.4996(9), N1–C1: 1.3578(17), C1–C2: 1.3809(17), C2–P1: 1.7654(14), P1–N2: 1.5847(13), N1–Fe1–N2: 107.11(5), Fe1–N2–Fe1’: 82.44(5), N2–Fe1–N2’: 97.55(5), C2–P1–N2: 113.94(7).
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phosphinimido unit without the intermediacy of a phosphinimine. Deuterium labeling experiments are consistent with a radical process. While the phosphazide moiety can act as a ligand, it is clear that it may also be a precursor to phosphinimido units. In the Staudinger process, it is the nitrogen that is attached to the azide unit that is retained upon thermolysis. In contrast, this hydride metathesis process results in the distal nitrogen to the organoazide being transferred to the phosphine.
Acknowledgements We thank NSERC of Canada for a Discovery grant to M. D. F. We also thank the Japan Society for the Promotion of Science for the grant “Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation” to T. O. and T. S. Scheme 3
Notes and references the caption in Fig. 3; these bond lengths are consistent with some delocalization around the N1–C13–C17–P1 unit. The measured magnetic moment for dimer 6 is 3.1 BM, which is consistent with two high spin Fe(II) centers antiferromagnetically coupled. The key transformation that has occurred is that the phosphazide unit has been converted into a phosphinimido unit via loss of N2 and xylene (1,3-Me2C6H4). When the reaction was performed with the corresponding deuteride, KBEt3D, no deuterium was detected in the xylene. However, when the reaction was performed in deuterated THF (THF-d8), 2-d1-xylene was generated as confirmed by GC-MS and 13C{1H} NMR spectroscopy (see ESI†). On the basis of these two experiments, we can rule out direct reaction of the hydride at the C–N bond of the coordinated phosphazide unit, or even intramolecular H-transfer from the Fe center to the ipso carbon of the arylazido unit, and instead invoke a radical process that generates an aryl radical at the 2-position of xylene, which subsequently abstracts a hydrogen or a deuterium from the solvent. One possible process is shown in Scheme 3, which involves the formation of the putative hydride 7, followed by elimination of H•, detected as H2 (or D2 for the reaction with KBEt3D),16 and a one-electron cascade that starts with a Fe(I) center and results in the generation of N2 and the xylyl radical;17 subsequent H or D abstraction from the protio or deutero solvent produces xylene or 2-d1-xylene, respectively. Once formed, the sterically unencumbered imine-phosphinimido iron(II) complex dimerizes to form 6.
Conclusions The surprising aspect of this work is not the isolation of a thermally stable phosphazide moiety that can also act as a ligand and remain intact upon coordination to iron(II); rather it is the hydride-induced conversion of the phosphazide unit into a
56 | Dalton Trans., 2015, 44, 54–57
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