Outer sphere coordination chemistry: Unusual six-coordinate silver(I) complexes with tri-halide–iridium(III) complexes as ligands

    Outer sphere coordination chemistry: unusual six-coordinate silver(I) complexes with tri-halide-iridium(III) complexes as ligands Thorbjørn J. Morsing, Kim P. Simonsen, Jesper Bendix PII: DOI: Reference:

S1387-7003(15)30025-3 doi: 10.1016/j.inoche.2015.07.013 INOCHE 6051

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Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

20 May 2015 17 July 2015 24 July 2015

Please cite this article as: Thorbjørn J. Morsing, Kim P. Simonsen, Jesper Bendix, Outer sphere coordination chemistry: unusual six-coordinate silver(I) complexes with tri-halide-iridium(III) complexes as ligands, Inorganic Chemistry Communications (2015), doi: 10.1016/j.inoche.2015.07.013

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Outer sphere coordination chemistry: unusual six-coordinate silver(I) complexes with tri-halide-iridium(III) complexes as ligands. Thorbjørn J. Morsing*,a, Kim P. Simonsenb and Jesper Bendix*,a

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a) Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Denmark b) School of Conservation, The Royal Danish Academy of Fine Arts, Esplanaden 34, DK-1263, Denmark

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Reaction of silver(I) with robust halido complexes of Ir(III) leads to second sphere coordination rather than halide abstraction. Two hexa-coordinated pseudo-octahedral silver(I) complexes [(CH3CN)3IrX3AgX3Ir(NCCH3)3]+ (X = Cl, Br) have been synthesized and structurally characterized as PF6- and SbF6- salts, respectively. The synthesis and crystal structure of the new, mono nuclear complex, [IrBr3(NCCH3)3], are also reported along with spectroscopic characterization of [IrX3(NCCH3)3] and the trinuclear silver complexes. The electronic structures are analyzed in the framework of ligand field theory and by comparison with high-level correlated multireference ab initio calculations. The ligand field strength of the heavier halide complexes is found to be significantly less sensitive to outer-sphere complexation as compared to the pronounced outer sphere ligation effects commonly observed for fluoride complexes.

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Abstraction of the heavier halide ligands using soft metal ions such as Ag+, Tl+, Pb2+, and Hg2+ is a classical approach in preparative coordination chemistry [1]. Conversely, the use of chloride, bromide, and iodide complexes as ligands towards soft metal centers is very little explored since the normal course of reaction when halide complexes are reacted with e.g. Ag(I) is exactly the halide abstraction and precipitation of insoluble silver halide. However, when the kinetically most robust metal centers are employed, like low-spin d6 Ir(III), sufficient kinetic barriers are available to suppress the halide abstraction and allow for second sphere coordination. In the following we describe the first systematic realization of this possibility, employing neutral, facial tri-halide complexes of Ir(III). Second sphere coordination of soft metal ions onto firmly bound chloride and bromide ligands offers the prospect of probing the sensitivity of the donor properties of these heavier halide ligands towards exterior perturbations. This is interesting in comparison with the properties of fluoride complexes, which have been shown to engage in second sphere coordination of hard metal ions and interactions with protic solvents leading to pronounced perturbations of ligand field parameters and to solvatochromism [2].

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The reaction of silver salts of weakly coordinating anions like AgOTf (OTf = trifluoromethylsulfonate), AgSbF6 and [Ag(NCCH3)4]PF6 with the tri-halo-tris(acetonitrile)iridium(III) complexes [IrCl3(NCCH3)3] (1) and [IrBr3(NCCH3)3] (2) in boiling acetonitrile, does not result in halide abstraction from the iridium center. Rather complex formation with the silver ions results as can be verified from mass spectrometry (vide infra). From the reaction mixtures, the two salts [(CH3CN)3IrCl3AgCl3Ir(NCCH3)3]PF6 (3) and [(CH3CN)3IrBr3AgBr3Ir(NCCH3)3]SbF6 (4) could be isolated in decent yields of 50-60%. The two compounds are iso-structural and crystallize in the rhombohedral space group R-3 with the Ir-Ag-Ir axis along the trigonal axis of the crystal (the c-axis) and with a central six-coordinate Ag centre. By virtue of good quality crystals, solvent free structures, high symmetry and a large number of heavy atoms, the obtained crystal structures are of high quality with R1 values of 0.87% and 1.14% for 3 and 4 respectively. ORTEP plots of the two structures are shown in Fig. 1 and selected metrical parameters given in the legend to the figure. The geometries around the silver ions are very similar in the two structures with X-Ag-X angles varying only 0.5° as is seen in the structure overlay in Fig 1b. Six-coordinate silver complexes are quite rare, especially with halide ligands, and complexes with TM-X-Ag connectivity (TM = transition metal, X = halide) with six-coordinate silver are very scarce. Only two other structures with this motif have been reported to date [3,4]. Compared to the structures of the mononuclear starting complexes, the [IrX3(NCCH3)3] moieties are changed very little upon complexation with the silver ion. The average Ir-Cl distance in 1 is 2.337(8) Å compared to 2.3391(4) Å in 3, and the Ir-Br distance in 2 (see Fig. 2) is on average 2.466(3) Å compared to 2.46859(16) Å in 4, thus showing no statistically significant change in Ir-X bond length upon complexation. The average Cl-Ir-Cl angle in 1 is 91.40(11)° compared to 89.885(14)° in 3, and *

Correponding Author Email adresses: [email protected], [email protected]

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the average Br-Ir-Br angle in 2 is 91.2(8)° compared to 90.185(6)° in 4, indicating a small contraction of the bite angle in the silver complexes. These very moderate structural perturbations could be taken to indicate that the coordination of the silver ion causes only a modest change in the donor properties of the halides. There is no structural evidence suggesting that the Ag-X interaction is weak in these systems. Thus, the Ag-X distances are at 2.7507(4) Å and 2.87320(17) Å in 3 and 4, respectively, slightly shorter than those found in the corresponding silver halide structures: 2.774 Å and 2.888 Å in AgCl and AgBr [5]. <>

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The Ag-X bond lengths are also close to those determined in the few other published structures of sixcoordinate silver(I) halide complexes. These are mainly constituted by complexes of the ligands dichloromethane and 1,2-dichloroethane and their bromide (and iodide) analogues, and of halo-borane complexes. Two notable exceptions, which have TM-Cl-Ag connectivity, more closely resembling the structural motif of 3 and 4, are the complexes [Ag([RhCl2(tacn)(NCCH3)])3]4+ [3] and cat([Ag(Mo10Cl35)]4-) [4]. All known six-coordinate silver(I) halide complexes/compounds and the average Ag-X bond length in these systems are listed in Table 1.

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The mono nuclear iridium precursor complex, 1, has been mentioned a few times in the literature [15] and its crystal structure has been reported [16], but the related 2 has not been described before. The structure for 2, which is accessible in 68 % yield from commercial IrBr3  4 H2O, is shown in Fig. 2 and the metrical details are given in the legend of that figure. <>

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Persistence of trinuclear 3 and 4 in acetonitrile solution was verified by electrospray mass spectrometry. In both cases, the molecular ions were observed along with fragments resulting from loss of one or two acetonitrile ligands or the loss of a [IrX 3(NCCH3)3] moiety, showing that the trinuclear complexes exist in solution (see Fig. S1 and S2 in the SI).

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Spectral comparison between Ir-(X-Ag) and Ir-X will allow for assessment of the change in donor strength suffered by the halide ligands by involvement in bridging to silver(I). The solution spectra and solid state reflectance spectra of 1 and 2 are shown in Figs. 3 and 4, while Gaussian deconvolutions of the spectra are given in the SI (cf. Fig. S4). Because of the facial conformation of the complexes 1 and 2 and the near orthoaxial disposition of the ligators, the ligand field is of holohedrized octahedral symmetry. In the following, the electronic states are, accordingly, discussed with symmetry labels belonging to the group O. The lowest energy transition observed in both complexes, 1 and 2, is the spin-forbidden 1A1  3T1 transition followed by the two spin-allowed transitions 1A1  1T1 and 1A1  1 T2. The second spin-forbidden transition (1A1  3T2), which must fall below the lowest spin-allowed transition, has an energy, which in the strong-field limit differs only by B/2 (for C/B = 4.25) from that of the first spin-allowed transition. Hence it cannot contribute to the spectral intensity below 30.000 cm-1. For the internal field strength (/B) in Ir(III) complexes, the lowest quintet state is well outside the observable range. As for the spin-allowed transitions(s), both 1A1  1T1 and 1A1  1T2 are easily identified in the spectrum of 1, while in the spectrum of 2, the transition at 36 700 cm-1 cannot be the 1 A1  1T2, as this would imply a significantly higher interelectronic repulsion in 2 compared to 1, contrary to established variation of nephelauxetism among the halides. Instead, the 1A1  1T2 transition in 2 is assigned to the component at 31800 cm-1 (cf. Fig. S4). While the presence of this component is difficult to unambiguously derive from the experimental spectra, as an almost equally good Gaussian decomposition can be achieved with one less component, its presence and position are corroborated by high-level multireference configuration interaction calculations in the form of MRDDCI2 calculations (vide infra). Additionally these calculations identify the higher lying absorption in 2 to encompass several LMCT transitions, which occur before the ‘wall’ of MLCT transitions at even higher energy (cf. Fig. 3). In the chloride complex this set of LMCT transitions is at higher energy and therefore hidden in under other more intense CT transitions. For a table containing the deconvoluted and computed peak positions, see the supporting information Table S1. <>

The reflectance spectra are interesting by their better resolution of the spin-forbidden transitions. In particular, at least two components can directly be observed at energies below 25.000 cm -1. Both of

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these have to originate from spin-orbit coupling splitting components of 3T1 and hence can provide a handle on the experimental determination of the spin-orbit coupling parameter in these systems.

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Since the coordination is very closely approximating orthoaxial, it is impossible to derive individual ligand field parameters for the halides and acetonitrile ligands – only averaged values can be determined. Fitting of the four/three d-d transitions observed for 1 and 2 to a non-additive ligand field parametrization employing full-matrix diagonalizations using the program LIGFIELD, [17] yields the parameter values  = 30400 cm-1; B = 376 cm-1 ; 5d = 2160 cm-1 for 1 and  = 28000 cm-1, B = 311 cm-1 ; 5d = 2270 cm-1 for 2, in both cases assuming C = 4.25B [18]. These values agree well with previously determined data for octahedral or pseudo octahedral Ir(III) complexes [19] High-level correlated multireference ab initio calculations are useful to unambigously assign mixed d-d and CT spectra. For this purpose, Multireference Difference-dedicated Configuration Interaction (MRDDCI) approaches has been advocated [20]. MRDDCI2 calculations on 1 and 2 overestimate the transition energies by a few thousand wavenumbers, but the relative positions of the transitions, also between species, are well reproduced (cf. Fig. 3). As is expected, the calculated intensities for d-d transitions are quite a bit too low because of the Laporte-forbidden nature of the transitions. In the experiment, they gain intensity because of vibrational breaking of the symmetry, which is not reproduced in the calculations. Here the intensity arises because the used experimental geometries do not possess perfect inversion symmetry (see SI, Fig. S3).

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The reflectance spectra of trinuclear 3 and 4 are practically indistinguishable from those of their mononuclear constituent complexes 1 and 2 in the d-d region, indicating no or very little change in the ligand field experienced by the Ir(III) centers upon outer sphere complexation with Ag(I) (cf. Fig. 4). This is supported by MRDDCI2 calculations on the dinuclear model, [(CH3CN)3IrX3Ag]+, which show shifts in the transition energies of less than 1000 cm-1 compared to those calculated for 1 (see supporting information Table S1).

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In conclusion, robust Ir(III)-halide complexes form outer sphere interactions with Ag +, leading to unusual six-coordinate silver complexes in the solid state. These complexes can also be detected in mass spectra of their solutions. Contrary to the strong perturbations in donor strength which fluoride ligands undergo when engaged in second sphere interactions, exemplified by the strong solvatochromism of many fluoride complexes, the ligand field experienced by Ir(III) is hardly affected by halide bridging to Ag+. In agreement with the vanishing structural effects of the complexation with Ag+, this may be interpreted as a practical insensitivity of the ligand field strength of chloride and bromide towards the perturbation by silver(I).

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Acknowledgement

J.B. acknowledges support from the Danish Research Councils for Independent Research for support (12-125226) References: [1]: (a) J. A. Davies, C. A. Hockensmith, V. Yu. Kukushkin, Yu. N. Kukushkin, Synthetic Coordination Chemistry: Principles and Practice, World Scientific Publishing, 1996 (b) W. Beck, K. Sünkel, Chem. Rev. 88 (1988) 1045-1421. [2]: (a) S. Kaizaki, H. Takemoto, Inorg. Chem. 29 (1990) 4960-4964 (b) Y. Terasaki, S. Kaizaki, J. Chem. Soc., Dalton Trans. (1995) 2837-2841. (c) Y. Terasaki, T. Fujihara, T. Schönherr, S. Kaizaki, Inorg. Chim. Acta 259 (1999) 84-90. (d) R. J. Bianchini, U.Geiser, H. Place,S. Kaizaki, Y. Morita, J. I. Legg, Inorg. Chem. 25(1986) 2129-2134. (e) T. Birk, M. J. Magnussen, S. Piligkos, H. Weihe, A. Holten, J. Bendix, J. Fluorine Chem. 131 (2010) 898906. [3]: M. Sudfeld, W. S. Sheldrick, Inorg. Chim. Acta, 304 (2000) 78-86. [4]: D. Freudenmann, C. Feldmann, Dalton Trans. 43 (2014) 14109-14113. [5]: W.G. Wyckoff , Crystal Structures Vol. 2, Second Edition, Interscience Publishers, 1964. [6]: A. Bihlmeier, M. Gonsior, I. Raabe, N. Trapp, I. Krossing, Chem.-Eur. J, 10 (2004) 5041-5051. [7]: D. M. Van Seggen, P. K. Hurlburt, O. P. Anderson, S. H. Strauss, J. Am. Chem. Soc., 114 (1992) 1099510997. [8]: I. Krossing, Chem.-Eur. J., 7 (2001) 490-502.

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[9]: K.-C. Kim, F. Hauke, A. Hirsch, P. D. W. Boyd, E. Carter, R. S. Armstrong, P. A. Lay, C. A. Reed, J. Am. Chem. Soc., 125 (2003) 4024-4025. [10]: D. M. Van Seggen, P. K. Hurlburt, O. P. Anderson, S. H. Strauss, Inorg. Chem., 34 (1995) 3453-3464. [11]: Z. Xie, R. Bau, C. A. Reed, Angew. Chem., Int. Ed., 33 (1995) 2433-2434. [12]: Z. Xie, B.-M. Wu, T. C. W. Mak, J. Manning, C. A. Reed, J. Chem. Soc., Dalton Trans. (1997) 1213-1218. [13]: Z. Xie, C.-W. Tsang, E. T.-P. Sze, Q. Yang, D. T. W. Chan, T. C. W. Mak, Inorg. Chem. 37 (1998) 64446451. [14]: I. Raabe, D. Himmel, S. Muller, N. Trapp, N. Kaupp, I. Krossing, Dalton Trans., (2008) 946-956. [15]: (a) F. Tessore, D. Roberto, R. Ugo, M. Pizzotti, S. Quici, M. Cavazzini, S. Bruni, F. De Angelis, Inorg. Chem., 44 (2005) 8967-8978. (b) T. J. Morsing, S. N. MacMillan, J. W. H. Uebler, T. Brock-Nannestad, J. Bendix, K. M. Lancaster, Inorg. Chem., 54 (2015) 3660-3669. [16]: G. J. Leigh, J. R. Sanders, P. B. Hitchcock, J. S. Dernandes, M. Togrou, Inorg. Chim. Acta, 330 (2002) 197212. [17]: S. Piligkos, J. Bendix, H. Weihe, C. J. Milios, E. K. Brechin, Dalton Trans, (2008) 2277-2284. [18]: J. Bendix, M. Brorson, C. E. Schaffer, Inorg. Chem. 32 (1993) 2838-2849. [19]: M. Brorson, M. R. Dyxenburg, F. Galsbøl, K. P. Simonsen, Acta Chem. Scand. 50 (1996) 289 – 293. [20]: (a) F. Neese, J. Am. Chem. Soc., 128 (2006) 10213-10222. (b) J. Miralles, O. Castell, R. Caballol, J.-P. Malrieu, Chem. Phys., 172 (1993) 33-43. (c) F. Neese, J. Chem. Phys., 119 (2003) 9428-9443.

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Fig. 1. Structures of the trinuclear units of 3 (a) and 4 (c) shown with 50% probability ellipsoids. Also shown (b) is an overlay between the two structures. Selected bond distances in Å and angles in ° for 3: Ag2-Cl1: 2.7507(4), Ir1-Cl1: 2.3391(4), Ir1-N4: 2.0014(13), P2-F3: 1.6085(11),

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Ir1-Cl1-Ag2: 81.828(11), Cl1-Ag2-Cl1∗: 106.160(14): Selected bond distances in Å and angles in ° for 4: Ag1Br1: 2.87320(17), Ir1-Br1: 2.46859(16), Ir1-N4: 2.00943(15), Sb1-F1: 1.87951(12), Ir1-Br1-Ag1: 80.493(2), Br1-Ag1-Br1∗: 105.036(2).

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Fig. 2. ORTEP plot of 2 shown with 50% probability ellipsoids. Selected bond distances in Å and angles in °: Ir1-Br1: 2.46333(16), Ir1-Br2: 2.46671(12), Ir1-Br3: 2.46851(13). Ir1-N1: 2.01031(14), Ir1-N2: 2.02002(11), Ir1-N3: 2.01896(10), Br1-Ir1-Br2: 91.726(3), Br1-Ir1-Br3: 91.591(3), Br2-Ir1-Br3: 90.236(5).

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Fig. 3. Experimental (top) and MRDDCI2 calculated spectra (bottom) of mononuclear 1 and 2.

Fig. 4. Kubelka-Munk representations of the reflectance spectra of 1-4.

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Table 1. Average Ag-X distances in known six-coordinate silver halide complexes or salts.

[5] [6] [7] [8] [9] [3] [4] [5] [10] [10] [11] [12] [13] [14]

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No metric data given because of inferior data-quality and high degree of disorder.

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AgCl [Ag(CH2Cl2)3]+ [Ag(CH2Cl2)3]+ [Ag(C2H4Cl2)3]+ [Ag(CH6B11Cl6)2]− [Ag([RhCl2(tacn)(NCCH3)])3]4+ cat-[Ag(Mo10Cl35)]4AgBr [Ag(CH2Br2)3]+ cat-[Ag(C2H4Br2)3]+ [Ag(CH6B11Br6)2]− cat-[Ag(CH6B11Br6)] cat-[Ag(B12Br12)] [Ag(CH2I2)3]+

Average Ag-X bond length 2.774 Å 2.752 Å 2.836 Å 2.742 Å 2.765 Å N/Aa 2.797 Å 2.888 Å 2.865 Å 2.886 Å 2.879 Å 2.862 Å 2.848 Å 3.050 Å

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Graphical abstract

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Highlights: "Robust Ir(III) allows for structural characterization of second-sphere complexation."

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"Unusual six-coordination of silver is accessible through chelation by metalloligands."

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"Less sensitivity of the donor properties of chloride and bromide as compared to fluoride towards external perturbations."

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