Amidino ligands obtained from the coupling of 1-methylcytosine and nitrile: a new method to incorporate biomolecules into luminescent Re(CO)3 complexes

Dalton Transactions COMMUNICATION This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2015, 00, 1-4 | 1 Please do not adjust margins Please do not adjust margins a. GIR MIOMeT-IU Cinquima-Química Inorgánica, Facultad de Ciencias, Campus Miguel Delibes, Universidad de Valladolid, 47011 Valladolid, Spain. E-mail: [email protected] † Electronic Supplementary Information (ESI) available: Synthesis and characterization of the complexes, NBO charges and Wiberg indexes for 1a , photophysical data, figure of the crystal structure of 2a , frontier molecular orbital compositions in the ground and excited states, and calculated excited energies and dominant orbital excitations from TD-DFT for 1a and 2a . CCDC 1415524-1415525. See DOI: 10.1039/x0xx00000x Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ Amidino ligands from coupling 1-methylcytosine and nitrile: a new method to incorporate biomolecules to luminescent Re(CO)3 complexes† Patricia Gómez-Iglesias,a Jose Miguel Martín-Alvarez,a Daniel Miguel,a and Fernando Villafañea,* The formation of an amidino chelating ligand from the coupling reaction of 1-methylcytosine and nitrile is the new method herein reported for the incorporation of biologically relevant substrates into rhenium(I) tricarbonyl complexes. The reactions are carried out thermally or microwave assisted. The use of luminescent rhenium(I) tricarbonyl complexes as labels and probes for biomolecules lies both on their intense and long-lived emission properties, and on the activity and binding selectivity of the biomolecules, which is retained in almost all cases.1 Therefore, the incorporation of biologically relevant substrates into these complexes is one of the most important challenges for future inorganic medicinal chemistry.1,2 More recently, IR spectroscopy has been used in these complexes to know the local environment without the need for labels or staining, allowing to combine bimodal IR and luminescent probes. This has been proposed and named as SCoMPIs, for "Single Core Multimodal Probe for Imaging", by Policar's group.3 Besides direct coordination of the nucleobase to the fac-Re(CO)3 fragment,4 three main strategies have been developed in order to graft biomolecules to the fac-[ReX(CO)3(N-N)]n (N-N = diimine chelating ligand; X = halogen or pseudohalogen, n = 0; X = pyridyl type ligand, n = +1) complexes:5 the biomolecule may be attached either to the diimine chelating ligand,6 or to the pyridyl type ligand;7 whereas the third option is attaching the biomolecule in a tripodal nitrogen-donor ligand on complexes fac-[Re(CO)3(N-N-N)] (N-N-N = tripodal nitrogendonor ligand).8 Herein we present a new method to incorporate biomolecules, in this case a nucleobase, to the rhenium(I) tricarbonyl moiety: instead of attaching a biomolecule to a chelating diimine previously coordinated, a new chelating ligand is formed by the reaction of the nucleobase with a coordinated nitrile. This process may be carried out thermally, or microwave assisted. This reaction is based on the activation of coordinated nitriles by the metal centre, which results in an enhancement of the electrophilicity of the carbon atom, and facilitates the addition of different nucleophiles.9 For instance, the addition of amines bearing a proton leads to amidines, of particular interest due to their organic, medicinal, or coordination chemistries.9c When the amine belongs to a heterocycle containing a donor atom in the appropriate position, the involvement of their electron pair in aromatization makes the resulting chelating amidino ligand significantly interesting. Our previous studies on pyrazole complexes10 led us to find that the formation of pyrazolylamidino complexes is base-catalysed,10e and to study their photochemistry10f or their properties as anion receptor.10b We envisaged a logical continuation of this previous work by attempting to make a new amidino complex from the reaction of a nucleobase and a rhenium(I) tricarbonyl nitrile precursor, since this reaction has not been previously reported for this metallic moiety, as indicated above. In fact, the field of metal ion-induced modifications to nucleobases is practically unexplored, although the coordination of nucleobases to metals has been profusely reported.4 The only precedent of metal-mediated transformations coupling reactions with nitriles and nucleobases is the amidino complex [ReCl4{NH=C(Me)(Me2AdH-  2N,N}], obtained after reaction of N6,N6-dimethyladenine (Me2AdH) with cis-[ReCl4(NCMe)2].11 The rest of processes of this type previously reported included the deprotonation of the nucleobase, affording an anionic chelating amidino ligand in the complexes cis-[L2Pt{NH=CR(MeAd-  2N,N}]+ (MeAdH = 9-methyladenine) or cis- [L2Pt{NH=CR(MeCy-  2N,N}]+ (L = PMePh2, PPh3; R = Me, Ph; MeCyH2 = 1-methylcytosine).12 COMMUNICATION Dalton Transactions 2 | Dalton Trans., 2015, 00, 1-4 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins The reactions of fac-[ReBr(CO)3(NCMe)2]13 with equimolar amounts of MeCyH2 in refluxing NCR (R = Me, Ph) lead cleanly to fac- [ReBr(CO)3{NH=C(R)(MeCyH-  2N,N}], (R = Me, 1a; R = Ph, 1b) as yellow microcrystalline solids (Scheme 1). The formation of the amidino chelating ligand by coupling of 1-methylcytosine and one molecule of acetonitrile is evident in the X-ray crystal structure of 1a, shown in Figure 1 together with selected distances and angles. Tables with details of the structure determination, and the rest of spectroscopic data for both complexes in accordance with the geometry deduced by x-ray diffraction can be found in the ESI. Scheme 1 Syntheses of the amidino complexes from coupling of MeCyH2 and NCR. Fig. 1 Perspective view of fac-[ReBr(CO)3{NH=C(Me)(MeCyH-  2N,N}], 1a, showing the atom numbering. Ellipsoids are drawn at 50 % probability. Selected bond lengths (Å) and angles (deg): Re1-N1 2.242(3), N1-C14 1.331(5), N3-C14 1.356(5), N3-C21 1.370(5), N4-C21 1.271(5), Re1-N4 2.132(3), N1-C11 1.409(5), N2-C11 1.400(4), N2-C12 1.346(5), C12-C13 1.322(6), C14-C13 1.417(5); C14-N1-Re1 125.5(2), N1-C14-N3 122.8(3), C14-N3-C21 133.3(3), N4-C21-N3 121.3(4), C21-N4-Re1 132.2(3), N4-Re1-N1 83.28(11). As indicated above, there are not previous reports of crystal structures containing amidino ligands derived from the coupling of nitriles and MeCyH2. The chelate six-membered ring containing the rhenium atom is almost planar, with a very slight distortion towards a boat conformation, where Re1 and N3 are 0.245(6) and 0.065(6) Å above the mean plane formed by N1, C14, C21 and N4. The latter mean plane forms an angle of 6.36(15)° with the cytosine ring, resulting in a twisted nucleobase ligand. Moreover, the mean plane of the whole nucleobase forms an angle of 12.79(12)° with the coordination plane defined by the C2, C3, N1, and N4 atoms. All these distorsions seem to be intended to move away the carbonyl group in the methylcytosine fragment from the carbonyl ligand in cis to the nitrogen donor atom of the nucleobase fragment. In fact, the O11-O3 and O11-C3 distances (2.948(4) and 2.591(6) Å) are well below the sum of the respective van der Waals radii, 3.04 and 3.22 Å, respectively). Obviously, the high steric crowding in this side of the molecule brings these two carbonyls apart from each other, and any chemical interaction between them should be discarded. The Re-N distances (2.242(3) and 2.132(3) Å) are similar to those previously found in pyrazolylamidino complexes,10 whereas two different C-N distances are found in the chelate six membered ring: those where C=N bonds may be proposed (N1-C14 1.331(5) and N4-C21 1.271(5) Å) are in the expected range for double C(sp2)=N(sp2) bonds,14 but the other CN distances (N3-C14 1.356(5) and N3-C21 1.370(5) Å) are shorter than those expected for a single C(sp2)N(sp2) bond.14 The tricoordinate N3 atom should be labelled as sp2 since it is planar, what implies that its electron pair should be delocalized. In order to support this, an NBO study was performed on the minimum geometry to calculate the Wiberg indexes of the bonds in the coordinated chelating ligand. The results, collected in Figure S1, support that the bond distances found in the crystal structure have an electronic origin and they are not due to packing effects. Therefore, the best description for this ligand is that depicted in Dalton Transactions COMMUNICATION This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2015, 00, 1-4 | 3 Please do not adjust margins Please do not adjust margins Scheme 1, although resonance forms where the C-N3 bonds have a double character also contribute to the resonance hybrid, as expected for the planar geometry of N3. Concerning this point, it should be pointed out that determining the energy of the possible tautomers is essential in biological processes, since those energetically less stable may be active intermediates for many transformations, what affects the mechanism of the processes where the biomolecule is involved.15 In fact, both the monodeprotonated cytosine anion, and the involvement of cytosine in hydrogen bonds or in coordination to metals have been theoretically evaluated.16 The N-bound hydrogen atom of the amidino ligand is involved in a hydrogen bond with the oxygen atom of a Me2CO molecule present in the crystal. The distances and angles detected (H(3)···O(91), 2.015(3) Å; N(3)···O(91) 2.874(4) Å, N(3)−H(3)···O(91) 176.6(3)°) leads to consider this hydrogen bond as "moderate".17 The new chelating ligands are robust enough so they remain unchanged when the complexes undergo further reactivity. Thus, the reactions of complexes 1 with AgBF4 in NCR afford the cationic complexes fac-[Re(CO)3(NCR){NH=C(R)(MeCyH-  2N,N}]BF4, (R = Me, 2a; R = Ph, 2b) after substituting the bromido ligand by NCR (Scheme 1). The crystallographic data for 2a may be found in the ESI, as well as their spectroscopic data. The distances and angles found in the crystal structure of 2a are very similar to those found for the structure of 1a, discussed above. These cationic complexes can also be obtained in a one-pot process from fac-[Re(CO)3(NCMe)3]BF4,18 1-methylcytosine, and the nitrile by a microwave assisted reaction, in 10 min at 180°C. The yields are slightly lower than those obtained when the reaction is carried out by traditional methods (76% vs. 92% for 2a, 60% vs. 83% for 2b). However, they are clearly higher than those once the yields of the necessary previous steps of 1a and 1b are considered (global yields 53% and 46% respectively, considering that the yield of both parent complexes fac-[ReBr(CO)3(NCMe)2] and fac-[Re(CO)3(NCMe)3]BF4 from fac-[ReBr(CO)5] are higher than 90% and therefore are almost quantitative). Therefore the microwave assisted reaction is a better synthetic method considering the whole atomic economy, since the microwave assisted processes start from fac-[Re(CO)3(NCMe)3]BF4. We are not aware of previous reports on the use of microwave to form amidines from nitriles and amines. However, microwave is not a suitable way to obtain the neutral bromido complexes 1, as the yields in this case are much lower than those obtained by refluxing the nitriles. As indicated above, the interest on these complexes lies on the incorporation of the nucleobase into a luminescent complex. It is well known that the Re(CO)3 complexes with chelate N-donor ligands are likely to be phosphorescent.19 In this way, we have recently described some similar complexes with pirazolylamidino ligands, and discussed which changes occur in the emission features when structural modifications are made.10f Nonetheless, we have measured some photophysical properties of compounds 1a and 2a, in order to check the luminescent behaviour of these nucleobase complexes. Their absorption spectra (see Figure S2 and Table S1 in the ESI) are very similar to those of pyrazolylamidino Re(CO)3 complexes.10f Thus, the intense bands observed in the UV region at high energy (250-320 nm) have an intraligand (IL) origin, while the lowest energy absorption bands are assigned to a mixture of MLCT Re→*(L), ligand-to-ligand chargetransfer (LLCT), and halide-to-ligand charge-transfer (XLCT) transitions. As expected, the substitution of the anionic -donor/-donor bromido ligand by a neutral -donor acetonitrile ligand led to an hypsochromic shift, in this case of ca. 60 nm in the low energy absorptions. Emission spectra showed bands in the range 500-580 nm, with quantum yields from 0.009 to 0.013 %, values that are in accordance with those found for the pirazolylamidino complexes. In order to support the assignment of the low-lying absorption transitions as MLCT, theoretical calculations at the same level of theory as for the pirazolylamidino complexes discussed above have been carried out for complexes 1a and 2a. These calculations showed that the highest occupied molecular orbitals (HOMOs) have a mixed Re/CO/Br character with different contributions in the case of the neutral complex 1a, while the HOMOs of the cationic complex 2a have a Re/CO character. In both cases the LUMO is mainly centred in the nucleobase ligand, confirming the metal to ligand charge transfer nature of the optical transitions (full details can be found in the ESI). In summary, new luminescent rhenium(I) tricarbonyl complexes containing amidino chelating ligands are obtained by coupling nitriles and 1-methylcytosine. The formation of new amidino chelating ligands in this system by extending this reaction to couple different nitriles and new nucleobases (besides cytosine, adenine and guanine contain donor atoms in the appropriate position to form new amidino ligands) is to be expected. Neutral and cationic complexes have been synthesized, the latter may also be obtained in a microwave reactor, which opens the door to the coordination of a wide range of substrates to the system. The authors wish to acknowledge Dr A. Kathyr (Université de Franche-Comté, France) for photophysical measurements, the Spanish Ministerio de Ciencia e Innovacin (CTQ2013-41067-P) for financial support, and P. G.-I. thanks the UVa for her grant. References 1 K. K.-W. Lo, K. Y. Zhang and S. P.-Y. Li, Eur. J. Inorg. Chem., 2011, 3551–3568. 2 D.-L. Ma, H.-Z. He, K.-H. Leung, D. S.-H. Chan and C.-H. Leung, Angew. Chem. Int. Ed., 2013, 52 , 7666–7682. 3 (a) S. Clède, F. Lambert, C. Sandt, Z. Gueroui, M. Refregiers, M.-A. Plamont, P. Dumas, A. Vessieres and C. Policar, Chem. Commun., 2012, 48 , 7729-7731. (b) S. Clède, N. Delsuc, C. Laugel, F Lambert, C. Sandt, A. Baillet-Guffroy and C. Policar, Chem. Commun., 2015, 51 , 2687-2689. 4 (a) T. A. Oriskovich, P. S. White, and H. H. Thorp, Inorg. Chem., 1995, 34 , 1629-1631. Some leading reviews: (b) P. Amo-Ochoa, F. Zamora, Coord. Chem. Rev., 2014, 276 , 34–58. (c) P. J. Bailey and S. Pace, Coord. Chem. Rev., 2001, 214 , 91–141. (d) B. Lippert, Coord. Chem. 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Dalton Transactions COMMUNICATION This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2015, 00, 1-4 | 5 Please do not adjust margins Please do not adjust margins COMMUNICATION Dalton Transactions 6 | Dalton Trans., 2015, 00, 1-4 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Electronic Supplementary Information Amidino ligands from coupling 1-methylcytosine and nitrile: a new method to incorporate biomolecules to luminescent Re(CO)3 complexes Patricia Gómez-Iglesias, Jose Miguel Martín-Alvarez, Daniel Miguel, and Fernando Villafañe* GIR MIOMeT-IU Cinquima-Química Inorgánica, Facultad de Ciencias, Campus Miguel Delibes, Universidad de Valladolid, 47011 Valladolid, Spain. Dalton Transactions COMMUNICATION This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2015, 00, 1-4 | 7 Please do not adjust margins Please do not adjust margins Synthesis and characterization of the complexes General Remarks. All manipulations were performed under N2 atmosphere following conventional Schlenk techniques. Solvents were purified according to standard laboratory methods. i fac-[ReBr(CO)3(NCMe)2], ii fac- [Re(CO)3(NCMe)3]BF4, iii and 1-methylcytosine iv were obtained as previously described. The microwave assisted reactions were carried out in an Anton Paar Monowave 300 apparatus. Infrared spectra were recorded in a Perkin-Elmer FT-IR spectrum BX apparatus using 0.2 mm CaF2 cells for solutions or in a Perkin-Elmer Frontier spectrometer coupled to a Pike GladiATR-210 accessory for solid samples. NMR spectra were recorded in Varian MR500 instrument at room temperature (r.t.), and are referred to the internal residual solvent peak for 1H and 13C{1H} NMR. Assignment of the 13C{1H} NMR data was supported by 2D HSQC and HMBC experiments and relative intensities of the resonance signals. UV-vis spectra were measured with a VARIAN-Cary 100 or Shimadzu UV-2550 spectrophotometers and emission spectra were recorded on a Jobin-Yvon FluoroLog 3.2.2 or in a Perkin–Elmer LS- 55 luminescence spectrometer at room temperature. The luminescence quantum yields Ø of the complexes were determined using cresyl violet as a luminescence quantum yield standard. v All measurements were performed in deaerated solvents. Elemental analyses were performed on a Perkin-Elmer 2400B microanalyzer. fac-[ReBr(CO)3{NH=C(Me)(MeCyH-  2N,N}], 1a. A solution of fac-[ReBr(CO)3(NCMe)2] (0.216 g, 0.5 mmol) and 1-methylcytosine (MeCyH2, 0.063 g, 0.5 mmol) in NCMe (20 mL) was stirred for 5 h at reflux. The volatiles were removed in vacuo and the yellow residue was crystallized in acetone/hexane at 20°C, giving a yellow microcrystalline solid, which was decanted, washed with hexane (3 x 3 mL approximately), and dried in vacuo, yielding 0.149 g (58 %). IR (THF, cm1): 2018 vs, 1912 vs, 1881 vs. IR (neat solid, cm1): 3462 m, 3225 m, 2027 vs, 1925 vs, 1902 vs, 1670 m, 1597 m, 1524 w, 1467 m, 1421 m, 1339 m, 1317 m, 1213 m, 1182 w, 1115 w, 1043 w, 806 w, 780 w, 650 w, 632 w, 552 w, 523 w. 1H NMR (499.7 MHz, CD3NO2): 2.41 (s, NH=CCH3, 3 H), 3.56 (s, CH3 MeCy, 3 H), 6.21 (d, J = 7.0 Hz, C5H MeCy, 1 H), 7.86 (d, J = 7.0 Hz, C6H MeCy, 1 H), 8.73 (s, NH MeCy, 1 H), 9.89 (s, COMMUNICATION Dalton Transactions 8 | Dalton Trans., 2015, 00, 1-4 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins NH=CCH3, 1 H). 13C{1H} NMR (125.7 MHz, CD3NO2): 24.3 (s, NH=CCH3), 40.5 (s, NCH3), 97.1 (s, C5H MeCy), 150.9 (s, C6H MeCy), 155.7 (s, CO MeCy), 161.6 (s, NH=CCH3), 162.1 (s, C4 MeCy), 197.1 (s, ReCO), 198.0 (s, ReCO), 198.5 (s, ReCO). Anal. Calcd. for C10H10BrN4O4Re: C, 23.26; H, 1.95; N, 10.85. Found: C, 22.99; H, 2.01; N, 10.69. fac-[ReBr(CO)3{NH=C(Ph)(MeCyH-  2N,N}], 1b. The same procedure as for 1a, using NCPh (7 mL) as solvent, gave 0.158 g (55%) of 1b as a yellow microcrystalline solid. IR (THF, cm1): 2018 vs, 1913 vs, 1884 vs. IR (neat solid, cm1): 3194 w, 2918 m, 2849 m, 2015 vs, 1918 s, 1894 vs, 1867 vs, 1680 m, 1575 m, 1509 m, 1455 m, 1442 m, 1415 m, 1332 m, 1303 w, 1254 m, 1174 w, 1127 w, 1041 m, 1024 w, 874 w, 797 w, 776 w, 697 m, 650 w, 624 w, 607 w, 556 w, 523 m, 481 w, 399 w, 360 w, 303 w. 1H NMR (499.7 MHz, CD3NO2): 3.61 (s, CH3 MeCy, 3 H), 6.34 (d, J = 7.0 Hz, C5H MeCy, 1 H), 7.6 (t, J = 7.5 Hz, meta-C6H5, 2 H), 7.69 (tt, J = 7.5 and 1.5 Hz, para-C6H5, 1 H), 7.77 (d, J = 7.5 Hz, ortho-C6H5, 2 H), 7.95 (d, J = 7.0 Hz, C6H MeCy, 1H), 8.89 (s, NH MeCy, 1 H), 9.12 (s, NH=CCH3, 1 H). 13C{1H} NMR (125.7 MHz, CD3NO2): 40.5 (s, NCH3), 97.5 (s, C5H MeCy), 128.2 (s, ortho-C6H5), 130.7 (s, meta- C6H5), 134.1(s, para-C6H5), 134.3 (s, ipso-C6H5), 151.1 (s, C6H MeCy), 155.8 (s, CO MeCy), 161.8 (s, N=CPh3), 162.8 (s, C4 MeCy), 197.2 (s, ReCO), 197.7 (s, ReCO), 198.4 (s, ReCO). Anal. Calcd. for C15H12BrN4O4Re: C, 31.14; H, 2.09; N, 9.69. Found: 30.93; H, 2.28; N, 9.89. fac-[Re(CO)3(NCMe){NH=C(Me)(MeCyH-  2N,N}]BF4, 2a. Method A. A mixture of 1a (0.103 g, 0.2 mmol) and AgBF4 (0.045 g, 0.23 mmol) in NCMe (20 mL) was stirred at 30°C for 30 min with exclusion of light. Then the reaction mixture was filtered, the volatiles were dried in vacuo, and the yellow residue was crystallized in THF/Et2O giving a pale yellow microcrystalline solid, which was decanted, washed with diethyl ether (3 x 3 mL approximately), and dried in vacuo, yielding 0.103 g (92 %). Method B. fac- [Re(CO)3(NCMe)3]BF4 (0.048 g, 0.10 mmol), 1-methylcytosine (0.012 g, 0.10 mmol), and NCMe (2 mL) were placed in a dry 10 mL glass vessel equipped with a magnetic stirbar. The vessel was sealed with a Dalton Transactions COMMUNICATION This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2015, 00, 1-4 | 9 Please do not adjust margins Please do not adjust margins septum and placed in the microwave apparatus, and heated at 180°C during 10 min. The reaction mixture was then cooled to 50 °C, and the contents were transferred into a schlenk flask, and the volatiles were removed in vacuo. Crystallization from THF/Et2O yielded 0.044 g (76 %) of 2a. IR (THF, cm1): 2031 vs, 1932 vs, 1912 vs. IR (neat solid, cm1): 3274 m, 2961 w, 2028 vs, 1949 w, 1899 vs, 1697 m, 1673 m, 1582 m, 1525 w, 1464 w,1418 w, 1346 w, 1313 m, 1260 w, 1188 w, 1078 vs, 1053 vs, 1004 vs, 878 w, 781 m, 709 w, 649 w, 626 m, 553 w, 536 w, 477 w, 399 w, 375 w. 1H NMR (499.7 MHz, CD3NO2): 2.35 (s, NCCH3, 3 H), 2.46 (s, NH=CCH3, 3 H), 3.59 (s, CH3 MeCy, 3 H), 6.28 (d, J = 7.0 Hz, C5H MeCy, 1 H), 7.95 (d, J = 7.0 Hz, C6H MeCy, 1 H), 8.73 (s, NH MeCy, 1 H), 9.02 (s, NH=CCH3, 1 H). 19F NMR (470.2 MHz, CD3NO2): 152.87 (s, 10BF4, 4 F), 152.92 (s, 11BF4, 4 F). 13C{1H} NMR (125.7 MHz, CD3NO2): 3.0 (s, NCCH3), 24.1 (s, NH=CCH3), 40.5 (s, NCH3), 97.1 (s, C5H MeCy), 122.8 (s, NCCH3) 151.6 (s, C6H MeCy), 155.9 (s, CO MeCy), 162.1 (s, NH=CCH3), 163.6 (s, C4 MeCy), 194.4 (s, ReCO), 195.5 (s, ReCO), 197.0 (s, ReCO). Anal. Calcd. for C12H13BF4N5O4Re: C, 25.54; H, 2.32; N, 12.41. Found: C, 25.63; H, 2.44; N, 12.15. fac-[Re(CO)3(NCPh){NH=C(Ph)(MeCyH-  2N,N}]BF4, 2b. Method A. A mixture of 1b (0.056 g, 0.1 mmol) and AgBF4 (0.023 g, 0.12 mmol) in THF (10 mL) was stirred at 30°C for 30 min with exclusion of light. Then the reaction mixture was filtered, the volatiles were dried in vacuo, and the yellow residue was redissolved in NCPh (3 mL) and stirred for 30 min. The volatiles were again dried in vacuo, and the yellow residue was crystallized in THF/Et2O giving a pale yellow microcrystalline solid, which was decanted, washed with diethyl ether (3 x 3 mL approximately), and dried in vacuo, yielding 0.057 g (83 %). Method B. The same microwave procedure as for 1b, using NCPh (2 mL) as solvent gave 0.041 g (60 %) of 2b. IR (THF, cm1): 2027 vs, 1920 vs, 1900 vs. IR (neat solid, cm1): 3293 m, 3262 m, 3196 w, 3112 w, 2027 vs, 1926 m, 1897 vs, 1654 m, 1567 m, 1506 w, 1492 w, 1454 m, 1447 m, 1406 m, 1338 w, 1302 w, 1243 w, 1131 w, 1054 s, 1025 s, 998 m, 809 w, 796 w, 779 w, 761 m, 699 m, 686 m, 633 m, 562 w, 529 m, 401 w, 363 w. 1H NMR (499.7 MHz, CD3NO2): 3.66 (s, CH3 MeCy, 3 H), 6.52 (d, J = 7.5 Hz, COMMUNICATION Dalton Transactions 16 | Dalton Trans., 2015, 00, 1-4 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins diffractometer fitted with an Atlas CCD detector. The crystals were kept at 293(2) K during data collection. Using Olex2, xix the structure was solved for complex 1a with the ShelXS structure solution program using direct methods, xx and with olex2.solve structure solution program using Charge Flipping for complex 2a,20 and then, the structures were refined with the ShelXL refinement package using least squares minimisation. xxi All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were set in calculated positions and refined as riding atoms, with a common thermal parameter. All graphics were made with Olex2, and distances and angles of hydrogen bonds were calculated with PARST xxii (normalized values). xxiii Dalton Transactions COMMUNICATION This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2015, 00, 1-4 | 17 Please do not adjust margins Please do not adjust margins Table S2. Frontier Molecular Orbital Compositions (%) in the Ground State for Complex 1a at the PBE1PBE Level Contribution (%) Orbital Energy (eV): Re: Br: CO: nucleob: main bond type HOMO-3 -7.27 8.10 32.61 2.57 56.73 p(Br) + (nucleob) HOMO-2 -6.80 64.02 7.42 24.69 3.87 d(Re) + (CO) HOMO-1 -6.42 44.64 32.45 19.79 3.12 d(Re) + p(Br) + (CO) HOMO -6.32 48.88 24.50 19.53 7.08 d(Re) + p(Br) + (CO) LUMO -2.15 0.97 0.07 2.82 96.14 *(nucleob) LUMO+1 -0.92 27.18 4.64 21.20 46.98 p(Re) + *(CO) + *(nucleob) LUMO+2 -0.54 12.37 1.49 30.90 55.24 p(Re) + *(CO) + *(nucleob) Table S3. Frontier Molecular Orbital Compositions (%) in the Ground State for Complex 2a at the PBE1PBE Level COMMUNICATION Dalton Transactions 18 | Dalton Trans., 2015, 00, 1-4 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Contribution (%) Orbital Energy (eV): Re: NCMe: CO: nucleob: main bond type HOMO-3 -7.89 7.56 1.11 2.42 88.91 (nucleob) HOMO-2 -7.37 68.88 0.98 25.95 4.19 d(Re) + (CO) HOMO-1 -7.22 58.19 3.90 24.43 13.48 d(Re) + (CO) HOMO -7.03 58.13 3.33 23.15 15.40 d(Re) + (CO) LUMO -2.57 0.85 0.00 2.79 96.52 *(nucleob) LUMO+1 -1.32 23.38 1.85 25.37 49.40 p(Re) + *(CO) + *(nucleob) LUMO+2 -1.18 33.48 22.26 43.01 1.24 p(Re) + *(NCMe) + *(CO) Table S4. Calculated Excited Energies, Dominant Orbital Excitations, and Oscillator Strength (f) from TD-DFT Calculations for Complex 1a state excitation Coef. Ecalc (eV) calc (nm) f exp (nm) Character S1 HOMO  LUMO HOMO-1  LUMO 0.66 0.25 3.23 384 0.0192 399 MLCT/LLCT/XLCT Dalton Transactions COMMUNICATION This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2015, 00, 1-4 | 19 Please do not adjust margins Please do not adjust margins S2 HOMO  LUMO HOMO-1  LUMO -0.25 0.66 3.35 370 0.0426 MLCT/LLCT/XLCT S5 HOMO-3  LUMO 0.68 4.23 293 0.1858 316 XLCT/ILCT Table S5. Calculated Excited Energies, Dominant Orbital Excitations, and Oscillator Strength (f) from TD-DFT Calculations for Complex 2a state excitation Coef. Ecalc (eV) calc (nm) f exp (nm) Character S1 HOMO  LUMO 0.69 3.50 354 0.0203 337 MLCT/LLCT S2 HOMO-1  LUMO 0.68 3.78 328 0.1331 308 MLCT/LLCT S4 HOMO-3  LUMO HOMO  LUMO+1 HOMO  LUMO+2 -0.35 -0.35 0.45 4.39 282 0.0441 MLCT/LLCT/ILCT S5 HOMO-3  LUMO HOMO  LUMO+2 0.48 0.44 4.44 279 0.1103 259 ILCT Table S6. Molecular orbital Compositions in the Excited States. Contribution (%) Complex Orbital Energy (eV): Re: NCMe: CO: nucleob: 1a HOMO -6.79 30.02 15.64 11.02 43.32 LUMO -3.83 1.57 0.05 3.01 95.38 2a HOMO -7.48 43.04 2.10 13.80 41.06 COMMUNICATION Dalton Transactions 20 | Dalton Trans., 2015, 00, 1-4 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins LUMO -4.28 14.05 0.56 4.76 80.62 Dalton Transactions COMMUNICATION This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2015, 00, 1-4 | 21 Please do not adjust margins Please do not adjust margins Table S7. Calculated Emission Energies and Dominant Orbital Emissions from TD-DFT Calculations. Complex state Excitation Coef. Ecalc (eV) calc (nm) exp (nm) Character 1a T1 HOMO  LUMO 0.94 1.71 725 501 3MLCT/3ILCT/ 2a T1 HOMO  LUMO 0.86 1.63 762 576 3MLCT/3ILCT/ i D. D. Perrin and W. L. F. Armarego, "Purification of Laboratory Chemicals"; 3rd ed.; Pergamon Press: Oxford, 1988. ii M. F. Farona and K. F. Kraus, Inorg. 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