Inorganic Chemistry Communications 27 (2013) 122–126

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Inorganic Chemistry Communications

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Synthesis and structure of vaulted trans-Bis[1-(2-phenoxy)-imidazol-2-ylidene-C2,O]platinum(II) complex

Naruyoshi Komiya ⁎, Atsushi Yoshida, Takeshi Naota ⁎Department of Chemistry, Graduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-8531, Japan

⁎ Corresponding authors. Fax: +81 6 6850 6222.E-mail addresses: komiya@chem.es.osaka-u.ac.jp (N

naota@chem.es.osaka-u.ac.jp (T. Naota).

1387-7003/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.inoche.2012.10.036

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 September 2012Accepted 26 October 2012Available online 1 November 2012

Keywords:Platinum(II)Vaulted complexCarbene complexSolid-state emission

trans-Bis[1-(2-phenoxy)-imidazol-2-ylidene-C2,O]platinum(II) complex bearing a dodecamethylene bridge(1) was synthesized and characterized by 1H, 13C NMR, COSY, NOESY, FT-IR, high-resolution mass spectros-copy, and single crystal X-ray diffraction. X-ray and 2D-NMR analyses revealed that complex 1 has a charac-teristic syn-conformation in both the solid and solution states, where cofacial phenoxy-NHC ligands arefolded and fixed under strong restriction by the vaulted structure. A crystal of 1 obtained from EtOH exhibitsweak blue phosphorescence under UV excitation at 320 nm.

© 2012 Elsevier B.V. All rights reserved.

The investigation of metallated N-heterocyclic carbenes (NHC) isan important area in the fields of organic and material chemistry. Avariety of metal carbene complexes with phenyl-, phenoxy-, andpyridyl-NHC ligands has been synthesized and extensively exploredfor the development of functional materials that exhibit both highcatalytic activity for molecular transformation [1] and characteristicemission properties [2–5].

Vaulted transition metal complexes with “picnic basket” shapedstructures have attracted much interest as potential functional mate-rials, due to their three-dimensional superstructures around bridgedligands and square planar coordination planes [6–8]. A variety of vault-ed complexes has been prepared by the reaction of square planar tran-sition metals with tether-linked ligands, including porphyrin [9],2,2-diiminoenamine [10], o-iminophenol [11], β-iminoketones [12],o-iminopyrrole [13], o-carbamoylpyridine [13b], 2,2′-bis(phosphino)-1,1′-biferrocene [14], and 1,3-bis(2-pyridylethynyl)benzene [15]. Thecorrelation between the structures and functions has been extensivelystudied in view of their inclusion properties [9d,10], molecular aggrega-tion [11b,c], optical properties [9e,11c,d], and catalytic activities forasymmetric transformation [9a–c,14]. Palladium NHC complexes withcrown ether-like bridges have been prepared and fully characterizedas vaulted bis-NHC transition metal complexes [16].

As part of our research program, which is focused on the develop-ment of transition metal-based functional materials, we have been in-vestigating new properties of trans-bis(o-phenoxyimino) d8 transition

. Komiya),

rights reserved.

metal vaulted complexes [11b–d]. Consequently, we have synthesizeda novel vaulted form of the trans-bis(o-phenoxy-NHC)platinum(II)complex (1), which has an extremely bent syn-conformation due tospecific restrictions caused by the vaulted structure. As o-phenoxy-NHC platinum(II) complexes, cis-ligating 2 [2a] and 3 [2b,c] havebeen reported. This is the first example of trans-analogs of thebis(o-phenoxy-NHC)Pt(II) complex. In this paper, we describe thesynthesis, structure, and solid-state emission properties of this complex.

Complex 1was prepared by the reaction of PtCl2(CH3CN)2 with anequimolar amount of bis(imidazolium) salt 5 in the presence ofNaOAc (2 equiv) in dimethyl sulfoxide (DMSO) at 80 °C. Imidazoliumsalt 5 was obtained from 1-(2-methoxyphenyl)imidazole 4 [17] and1,12-dibromododecane (Scheme 1). Complex 1 was characterizedby 1H and 13C nuclear magnetic resonance spectroscopy (NMR), Fou-rier transform infrared (FT-IR) spectroscopy, high-resolution massspectroscopy (HRMS), and H–H correlation spectroscopy (COSY)and Nuclear Overhauser effect spectroscopy (NOESY) [18].

Trans-coordination and vaulted structures of 1 have been unequiv-ocally established by X-ray diffraction analysis of crystals obtained byrecrystallization from EtOH [19]. An ORTEP drawing of 1 is shown inFig. 1. Complex 1 has an extremely bent syn-conformation, where the

Scheme 1. Synthesis route for dodecamethylene vaulted trans-bis(o-phenoxy-NHC)platinum(II) 1.

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dihedral angles between the coordination [Pt(1)–O(1)–C(1)/Pt(1)–O(2)–C(10)] and aromatic planes [C(4)–C(6)–C(8)/C(13)–C(15)–C(17)] are 130 and 147° (Fig. 1b). Large torsion angles of [C(1)–Pt(1)–O(1)–C(4)] (59.7°) and [C(10)–Pt(1)–O(2)–C(13)] (42.4°) indicate thatthe bent structure is mainly due to the rotation of phenoxy moietiesthrough the Pt(1)–O(1) and Pt(1)–O(2) bonds. The NHC moieties alsohave twisted coordination with torsion angles of 23.0° [O(2)–Pt(1)–C(1)–N(1)] and 25.3° [O(1)–Pt(1)–C(10)–N(3)]. Doubly bridgedH-bonding interactions between phenoxy O(1)/O(2) and linker H(30)/(H19) adjacent to the other NHC moiety are observed with sufficiently

Fig. 1. ORTEP drawing of 1. (a) Top view. (b) Side view. Hydrogen atoms H(19) and H(30) ware shown at the 30% probability level. Dihedral angles between the coordination [Pt(1)–O(1are given in parentheses. Selected bond distances (Å) and angles (deg): Pt(1)–O(1), 2.022(1178.5(12); O(1)–Pt(1)–O(2), 175.6(8); C(1)–Pt(1)–O(1)–C(4), 59.7(14); C(10)–Pt(1)–O(2)N(2)–C(5)–C(4), 23(5); C(10)–N(4)–C(14)–C(13), 32(5).

Fig. 2. Molecular structure of cis-bis(o-phenoxy-NHC)platinum(II) complex 2 [2a]. (a) Top vO(2)–C(10)] and aromatic planes [C(4)–C(6)–C(8)/C(13)–C(15)–C(17)] are given in paren

short distances of 2.51 and 2.42 Å, respectively. It is noteworthy thatnon-vaulted analogs of bis(o-phenoxy-NHC)platinum(II) complex 2are selectively formed in cis-coordination with anti-conformation, asshown in Fig. 2 [2a]. Thus, we can be fairly certain that the present spe-cific trans-coordination with syn-conformation is not due to electronicinfluence on the ligand field, but from steric restriction caused by thevaulted structure.

The behavior of complex 1 in the solution state was examined byNMR analysis. The 1H NMR (500 MHz) spectrum (Fig. 3) of 1 in CDCl3exhibits two ddd signals at 3.77 and 5.63 ppm, which are assigned togeminal protons of bridging methylene carbons adjacent to nitrogenatoms of NHC [H(19a)/H(30a) and H(19b)/H(30b)]. Clear splittingand extreme downfield shift of H(19b)/H(30b) protons indicatethat these protons undergo H-bonding interaction with O(2)/O(1), res-pectively, similar to those in the crystalline state (Fig. 1). A NOESY(500 MHz) spectrumof a solution of 1 in CDCl3 is shown in Fig. 4. Strongremote correlation is observedbetween theH(19b) andH(18)protons onthe other side of the aromatic region (correlation A in Fig. 4) as well asexpected correlations of H(2)–H(19a)/H(11)–H(30a) (B), H(2)–H(3)/H(11)–H(12) (C), H(3)–H(6)/H(12)–H(15) (D), H(6)–H(7)/H(15)–H(16) (E), and H(7)–H(8)/H(16)–H(17) (F). This is a good indicationthat the bent syn-conformation of 1 is maintained in the solution statedue to steric restriction by the dodecamethylene bridge.

ere refined in calculated positions using appropriate riding models. Thermal ellipsoids)–C(1)/Pt(1)–O(2)–C(10)] and aromatic planes [C(4)–C(6)–C(8)/C(13)–C(15)–C(17)]8); Pt(1)–O(2), 2.04(2); Pt(1)–C(1), 1.99(3); Pt(1)-C(10), 2.03(3); C(1)–Pt(1)–C(10),–C(13), 41.5(18); O(2)–Pt(1)–C(1)–N(1), 23(3); O(1)–Pt(1)–C(10)–N(3), 26(3); C(1)–

iew. (b) Side view. Dihedral angles between the coordination [Pt(1)–O(1)–C(1)/Pt(1)–theses.

Fig. 3. 1H NMR spectrum of 1 (500 MHz, CDCl3, 298 K).

Fig. 4. Aromatic region of the 500 MHz 1H NOESY specturm of 1 in CDCl3 (298 K, mixing time=0.700 s).

124 N. Komiya et al. / Inorganic Chemistry Communications 27 (2013) 122–126

Fig. 5. Emission spectrum of crystal 1 (λex=320 nm, 77 K).

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Colorless plates of complex 1 obtained by recrystallization fromEtOH exhibit a weak blue phosphorescent emission (λmax=435,460 nm, λex=320 nm) at 77 K, although a solution of 1 in CDCl3 is al-most non-emissive. The emission spectrum of crystal 1 is shown inFig. 5. Tetradentate analog 3 exhibits an intense blue emission bothin the solution and solid states [2b]. The weak emission propertiesof 1 are due to the highly bent conformation of the coordinationsite, which collapses the d-π conjugation of the potentially emissiveplatform.

In summary, the first trans-ligating bis(o-phenoxy-NHC)Pt(II)complex was prepared by utilizing high conformational restrictiondue to the vaulted structure. The extremely bent syn-conformation ismaintained in both the solid and solution states, as confirmed byX-ray diffraction and 2D-NMR analyses. Research is currently underwayto apply the related vaulted complexes as new functional materials.

Acknowledgment

This work was supported by Grant-in-Aid for Scientific Research,from theMinistry of Education, Culture, Sports, Science and Technology,Japan.

Appendix A. Supplementary data

CCDC 901414 contains supplementary crystallographic data for 1.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge CrystallographicData Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2012.10.036.

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[18] Characterization of 1: IR (KBr) 2925, 2852, 1736, 1594, 1562, 1493, 1455, 1424,1402, 1298, 1128, 1039, 953, 849, 744, 693 cm−1; 1H NMR (CDCl3, 500 MHz) δ

1.20–1.52 (m, 16 H), 2.02–2.08 (m, 4 H, –NCH2CH2), 3.77 (ddd, J=13.6, 5.9,5.9 Hz, 2 H, H19a), 5.63 (ddd, J=13.6, 7.3, 7.3 Hz, 2 H, H19b), 6.60 (ddd, J=8.1,6.2, 2.2 Hz, 2 H, H7), 6.90 (dd, J=8.2, 2.2 Hz, 2 H, H9), 6.93 (ddd, J=8.2, 6.2,1.2 Hz, 2 H, H8), 7.02 (d, J=2.1 Hz, 2 H, H2), 7.17 (dd, J=8.1, 1.2 Hz, 2 H, H6),7.43 (d, J=2.1 Hz, 2 H, H3); 13C NMR (CDCl3, 125 MHz) δ 165.1, 159.0, 129.9,126.7, 121.9, 120.8, 119.1, 115.7, 115.6, 48.1, 30.6, 27.8, 27.6, 27.5, 25.4; HRMS(FAB+) m/z calcd for C30H36N4O2

195Pt(M)+: 679.2486; found: 679.2471. Assign-ment of 1H signals was determined by H–H COSY and NOESY experiments.

[19] Crystallographic data for 1: C32H42N4O3Pt, Mr=725.80, Monoclinic, P21/c,a=10.728(2), b=10.017(2), c=27.434(4) Å, β=90.180(4)°, V=3388(2) Å3,Z=4, Dc=1.635 g/cm3, μ=82.632 cm−1, F(000)=1456.00, GOF=1.14. R1(I>2σ(I))=0.18, wR2 (all reflns)=0.44.