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Article Cite This: Inorg. Chem. 2018, 57, 9364−9375

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Dimolybdenum Paddlewheel as Scaffold for Heteromultimetallic Complexes: Synthesis and Photophysical Properties Nicolai D. Knöfel,† Caroline Schweigert,‡ Thomas J. Feuerstein,† Christoph Schoo,† Niklas Reinfandt,† Andreas-Neil Unterreiner,*,‡ and Peter W. Roesky*,† †

Institute of Inorganic Chemistry and ‡Institute of Physical Chemistry, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany

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S Supporting Information *

ABSTRACT: A diphenylphosphine functionalized benzoic acid was applied for the synthesis of a homoleptic dimolybdenum-based metalloligand, exhibiting four symmetrically placed phosphine donor sites. This allowed subsequent treatment with gold(I), rhodium(I), and iridium(I) precursors to obtain early−late heterometallic complexes as well as Lewis acid−base adducts with BH3. The compounds were in-depth investigated by spectroscopic techniques, single-crystal X-ray diffraction, and femtosecond laser spectroscopy. The coordination of different metal fragments to the dimolybdenum metalloligand leads to a finetuning of the system’s optical properties, which correlates well with fluorescence quantum yield measurements. Nevertheless, triplet dynamics still remain the dominating channel in these systems with an intersystem crossing time constant below 1 ps.

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INTRODUCTION In 1964, X-ray diffraction measurements of molybdenum(II) acetate enabled the structural characterization of what later proved to be the first molybdenum−molybdenum quadruple bond.1 Four acetate groups act as bidentate bridging ligands, forming a homoleptic paddlewheel structure around a dimolybdenum unit,2 prototypical for most dimolybdenum(II) tetracarboxylate complexes. Ever since, molybdenum received considerable attention for the formation of multiply bonded M2n+ compounds, due to its easy access, unique scaffold, redox behavior, spectroscopic properties,3 as well as catalytic applications.4−6 Two well-established ligand systems to support a bridged dimolybdenum paddlewheel structure, via a bidentate O,O- or N,N-coordination, are carboxylates (O2CR−) and amidinates ((RN)2C(R′)−).3 The judicious design of these ligands does not only help to adjust the solubility behavior and the steric demand of metal complexes, yet it further enables the introduction of additional functionalities. Most notably, the synthesis and stabilization of the first dimolybdenum quintuple bond in 2009 was achieved by application of highly steric amidinate ligands.7 To selectively build up heteromultimetallic molecular structures containing a dimolybdenum unit, the application of bifunctional ligand systems, exhibiting orthogonal functionalities, is essential. This was, for example, demonstrated by Mashima et al., who utilized 6-diphenylphosphino-2-pyridonate (pyphos) ligands to build-up predefined heterometallic structures with Pd(II) and Pt(II), exhibiting a Mo24+ core unit.8 Similarly, the application of phosphinefunctionalized carboxylic acids seems advantageous, as they are © 2018 American Chemical Society

able to selectively coordinate different metal ions by their hard carboxylic and their soft phosphine donor centers and have already proven useful for the synthesis of early late heteromultimetallic complexes. The combination of early and late transition metals within one organometallic compound has been extensively studied in the last years.9 Most interestingly, cooperative effects, caused by electronic communication between metal ions, can lead to advanced catalytic systems as well as interesting photophysical properties.10 Thus, the formation of heterometallic complexes, combining a quadruply bonded dimolybdenum unit with late transition metals, seems exciting, especially regarding their tunable photophysical properties. There already have been several theoretical and experimental studies on the photophysical properties of Mo24+ quadruply bonded complexes. According to this, the bimetallic tetracarboxylates Mo2(O2CR)4 are well-known for their typical paddlewheel structure in a local D4h symmetry, with the Mo2bonding configuration σ2π4δ2.3,11 Earlier studies of these complexes, which addressed the electronic δ−δ* transition, were considerably hampered by a poor overlap between the δorbitals of the metal, resulting in accordingly low intensities of the corresponding transition (extinction coefficient ε ≈ 100 L· mol−1·cm−1).3,12,13 In contrast, the electronic transition between M2δ and Lπ* orbitals represents a fully allowed and bright metal-to-ligand charge transfer (MLCT) (ε ≈ 10 000− 14 000 L·mol−1·cm−1). This MLCT can be tuned over a very Received: May 16, 2018 Published: July 25, 2018 9364

DOI: 10.1021/acs.inorgchem.8b01334 Inorg. Chem. 2018, 57, 9364−9375

Inorganic Chemistry

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wide range of 400−1200 nm by appropriate selection of πacceptor ligands (O2CR; (RN)2C(R′)) and transition metals (Mo; W).14−16 However, these bright MLCTs tend to mask or, at least, considerably complicate investigations of pure 1 (δ−δ*) transitions.14,17 Consequently, many time-resolved investigations of Mo2(O2CR)4 compoundswith femtosecond or nanosecond time resolutionfocused on excitations of the 1MLCT state, which conveniently allowed observations of subsequent relaxation processes. As a result, lifetimes of the 1 MLCT state between 1 and 20 ps were reported. This relaxation is often superimposed by intersystem crossing (ISC) to a metal-centered triplet state, typically termed 3Mo2δδ* in the case of dimolybdenum carboxylate complexes, making an elucidation of ISC rates cumbersome.15,18,19 The corresponding lifetimes of this triplet state are typically between 1 and 100 μs, unaffected by the electronic structure of the ligands. Further depopulation channels are known to occur via radiative processes such as fluorescence and phosphorescence.15,20 Recently, interest has focused on charge distribution after MLCT excitation within the first electronically excited state and the influence by the electronic structure of the ground state.15,21 This is particularly important, considering potential applications in optoelectronics, in which the extent of charge delocalization over the ligand correlates with the optical properties. Lifetimes of excited 1MLCT for complexes like [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) and Pt(II) acetylides are usually below 100 fs and thus considerably shorter than typical M2-quadruply bonded compounds.15,20,22−25 Detailed studies are in need for an in-depth understanding of an underlying mechanism. Herein, we report the convenient synthesis and characterization of dimolybdenum complex [Mo2(p-O2CPhPPh2)4] (1) as a fourfold metalloligand, applying the bifunctional ligand 4(diphenylphosphino)benzoic acid (Scheme 1). The four

Article

EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under exclusion of moisture and oxygen in flame-dried Schlenk-type glassware or in an argon-filled MBraun glovebox. Prior to use, dimethyl sulfoxide (DMSO) was distilled under nitrogen from CaH2; likewise, methanol was distilled from magnesium, and dichloromethane was distilled from CaH2. Hydrocarbon solvents (tetrahydrofuran (THF), n-pentane) were dried using an MBraun solvent purification system (SPS-800). THF was additionally distilled under nitrogen from potassium before storage over 4 Å molecular sieves. Deuterated solvents were obtained from Carl Roth GmbH (99.5 atom % D). Prior to use, d8-THF was stored over a Na/K alloy, and CDCl3 was stored over molecular sieves (4 Å). NMR spectra were recorded on a Bruker Avance II 300 MHz or Avance 400 MHz. 1H and 13 C{1H} NMR chemical shifts were referenced to the residual 1H and 13 C resonances of the deuterated solvents and are reported relative to tetramethylsilane (TMS). 31P{1H} and 19F{1H} NMR resonances were referenced to external 85% phosphoric acid and CFCl3, respectively. 11B NMR resonances were referenced to 15% BF3· Et2O in CDCl3. IR spectra were obtained on a Bruker Tensor 37 FTIR spectrometer equipped with a room temperature DLaTGS detector and a diamond attenuated total reflection (ATR) unit. Raman spectra were obtained on a Bruker MultiRam spectrometer. Elemental analyses were performed with an Elementar Micro Cube. Absorption spectra were recorded by a UV/vis spectrometer Cary 500 (Varian), and fluorescence spectra were obtained by a fluorescence spectrometer FluoroMax 4 (HORIBA Scientific). Fluorescence quantum yields (QY) were determined by means of the wellestablished standard Rhodamine 6G (QY ≈ 0.95) in ethanol (99.5%).26,27 The femtosecond time-resolved pump−probe measurements and the principle experimental setup were described elsewhere.28,29 Relevant modifications of the transient absorption (TA) spectrometer are shown in the Supporting Information. [Mo2(OAc)4],30 K4[Mo2Cl8],31 as well as [AuCl(tht)]32 and [AuC6F5(tht)]33 (tht = tetrahydrothiophene) were prepared according to literature procedures. Bis(1,5-cyclooctadiene)diiridium(I) dichloride (97%), molybdenum hexacarbonyl (98%), and borane tetrahydrofuran complex solution (1.0 M in THF) were purchased from Sigma-Aldrich. 4-(Diphenylphosphino)benzoic acid (97%) was purchased from Acros Organics (97%), and bis(1,5-cyclooctadiene)dirhodium(I) dichloride was purchased from Alfa Aesar. They were all used as received. Synthesis. General Information. Synthesis in THF or DMSO leads to an axial solvent coordination to the Mo24+ unit, which cannot always be totally removed in vacuum. Thus, residues of THF and DMSO can be observed in the corresponding NMR, IR, and Raman spectra as well as elemental analyses. All spectra are given in the Supporting Information (Figures S34−S71). Synthesis of [Mo2(p-O2CPhPPh2)4] (1). In a modified procedure,34 molybdenum(II) acetate (82.6 mg, 0.19 mmol, 1.00 equiv) and 4(diphenylphosphino)benzoic acid (266 mg, 0.87 mmol, 4.50 equiv) were dissolved in THF (15 mL) and stirred at room temperature for 16 h. The solution was concentrated under reduced pressure, and after addition of n-pentane (30 mL) an orange solid precipitated. The residue was filtered off, washed with methanol (10 mL), and dried under vacuum. Single crystals suitable for X-ray analysis were obtained after slow cooling of a hot dimethyl sulfoxide solution or slow diffusion of n-pentane into a solution of 1 in THF. Crystalline yield: 226 mg (83%). 1 H NMR (d8-THF, 300 MHz): δ [ppm] = 8.27−8.13 (m, 8H, Ph), 7.40−7.25 (m, 48H, Ph). 13C{1H} NMR (d8-THF, 75 MHz): δ [ppm] = 177.1 (CO2R), 143.5 (d, 1JC,P = 14.2 Hz, i-CP), 138.2 (d, 1 JC,P = 11.9 Hz, i-CP), 134.8 (d, 2JC,P = 20.0 Hz, o-CPh), 134.3 (d, 2JC,P = 19.3 Hz, o-CPhCOO), 132.4 (i-CPhCOO), 131.0 (d, 3JC,P = 6.5 Hz, mCPhCOO), 129.9 (p-CPh), 129.6 ppm (d, 3JC,P = 7.0 Hz, m-CPh). 31 1 P{ H} NMR (d8-THF, 121 MHz): δ [ppm] = −5.18 (s, PPh2Ar). IR (ATR): ν̃ [cm−1] = 3067 (vw), 3056 (vw), 3022 (vw), 1595 (m), 1584 (w), 1568 (m), 1493 (s), 1433 (m), 1392 (s), 1305 (w), 1274 (w), 1245 (w), 1207 (vw), 1182 (m), 1157 (vw), 1142 (w), 1112

Scheme 1a

a

Typical coordination motifs of the bifunctional ligand 4(diphenylphosphino)benzoic acid (A); dimolybdenum(II) metalloligand (1) allows the subsequent buildup of heteromultimetallic complexes via its free phosphine donor sites (B).

symmetrically placed phosphine donor sites allow for subsequent treatment with late transition metals, obtaining early−late heterometallic complexes as well as Lewis acid−base adducts. The dimolybdenum scaffold and the influence of additionally coordinated metal complexes on the system’s photophysical properties was investigated in depth by advanced photophysical studies. 9365

DOI: 10.1021/acs.inorgchem.8b01334 Inorg. Chem. 2018, 57, 9364−9375

Article

Inorganic Chemistry (w), 1086 (m), 1068 (w), 1027 (m), 1017 (m), 999 (m), 919 (w), 910 (w), 850 (m), 765 (s), 742 (s), 719 (s), 694 (vs), 557 (w), 541 (w), 505 (s), 485 (s), 435 (s), 419 (s). Raman (solid state): ν̃ [cm−1] = 3058 (vw), 1596 (s), 1497 (m), 1470 (vw), 1410 (vs), 1393 (s), 1184 (m), 1087 (w), 1029 (vw), 1019 (vw), 1000 (w), 729 (w), 687 (vw), 634 (vw), 619 (vw), 563 (m), 504 (vw), 489 (vw), 415 (vw), 399 (m), 315 (vw), 306 (w), 269 (w), 253 (w). Anal. Calcd (%) for [C76H56Mo2O8P4] (1413.09 g·mol−1): C 64.60, H 3.99; found C 64.53, H 3.91. Synthesis of [Mo2(p-O2CPhPPh2)4(AuC6F5)4] (2). The phosphinefunctionalized metalloligand 1 (50.0 mg, 0.04 mmol, 1.00 equiv) and [AuC6F5(tht)] (64.0 mg, 0.14 mmol, 4.00 equiv) were dissolved in dichloromethane (DCM; 10 mL) and stirred at room temperature for 4 h. The solvent of the dark red solution was removed under reduced pressure; the obtained red solid washed with n-pentane (10 mL) and dried under vacuum. Single crystals suitable for X-ray analysis were obtained from slow diffusion of n-pentane into a solution of 2 in THF. Yield (single crystals): 70.1 mg (69%). 1 H NMR (d8-THF, 300 MHz): δ [ppm] = 8.48−8.31 (m, 8H, Ph), 7.81−7.49 (m, 48H, Ph). Additional resonances (m) for residually coordinated THF are observed at 3.67−3.59 and 1.84−1.74 ppm. 13 C{1H} NMR (d8-THF, 75 MHz): δ [ppm] = 176.5 (d, 5JC,P = 1.0 Hz, CO2R), 135.43 (d, JC,P = 13.9 Hz, CHPh), 135.36 (d, JC,P = 13.6 Hz, CHPh), 135.1 (d, 1JC,P = 53.3 Hz, i-CP), 134.4 (d, 4JC,P = 2.2 Hz, p-CPh), 133.0 (d, 4JC,P = 2.2 Hz, p-CPh), 131.7 (d, JC,P = 11.4 Hz, CHPh), 130.6 (d, 1JC,P = 55.2 Hz, i-CP), 130.5 (d, JC,P = 11.3 Hz, CHPh). Carbon atoms of the C6F5 ligands cannot be observed. 31 1 P{ H} NMR (d8-THF, 121 MHz): δ [ppm] = 42.1 (pseudoquintet, P-AuC6F5). 19F{1H} NMR (d8-THF, 282 MHz): δ [ppm] = −116.5 (m, 8F, o-F), −160.4 (t, 3JF,F = 19.7 Hz, 4F, p-F), −164.0 (m, 8F, mF). IR (ATR): ν̃ [cm−1] = 1634 (w), 1600 (vw), 1571 (vw), 1501 (s), 1454 (s), 1436 (s), 1393 (s), 1355 (m), 1308 (w), 1259 (w), 1185 (w), 1161 (vw), 1143 (vw), 1100 (m), 1073 (m), 1059 (m), 1052 (m), 1028 (vw), 1016 (w), 999 (w), 953 (vs), 847 (m), 792 (m), 764 (m), 746 (m), 725 (s), 711 (m), 691 (s), 633 (w), 618 (w), 602 (vw), 567 (w), 529 (s), 505 (s), 490 (s), 459 (w), 427 (m). Raman (solid state): ν̃ [cm−1] = 3058 (vw), 1599 (s), 1498 (m), 1404 (vs), 1186 (m), 1096 (w), 1029 (vw), 1018 (vw), 1001 (w), 735 (w), 700 (vw), 634 (vw), 620 (vw), 574 (m), 531 (vw), 490 (w), 425 (vw), 397 (m), 358 (vw), 279 (m). Anal. Calcd (%) for [C100H56Au4F20Mo2O8P4· (C4H8O)] (2941.30 g·mol−1): C 42.47, H 2.19; found C 42.18, H 2.36. Synthesis of [Mo2(p-O2CPhPPh2)4(RhCl(cod))4] (3). The phosphine-functionalized metalloligand 1 (49.0 mg, 0.03 mmol, 1.00 equiv) and bis(1,5-cyclooctadiene)dirhodium(I) dichloride (34.2 mg, 0.07 mmol, 2.00 equiv) were dissolved in THF (10 mL) and stirred at room temperature for 16 h. An orange solid precipitated. The mother liquor was decanted off, and the product was recrystallized from hot THF, washed with n-pentane (5 mL), and dried under vacuum. Crystalline yield: 41.7 mg (50%). 1 H NMR (CDCl3, 400 MHz): δ [ppm] = 8.28−8.17 (m, 8H, Ph), 7.82−7.68 (m, 24H, Ph), 7.45−7.33 (m, 24H, Ph), 5.64−5.54 (m, 8H, COD−CH), 3.21−3.11 (m, 8H, COD−CH), 2.48−2.30 (m, 16H, COD−CH2), 2.14−2.01 (m, 8H, COD−CH2), 1.98−1.87 (m, 8H, COD−CH 2 ). Additional resonances (m) for residually coordinated THF are observed at 3.76−3.66 and 1.87−1.80 ppm. 13 C{1H} NMR (CDCl3, 101 MHz): δ [ppm] = 176.5 (CO2R), 136.5 (d, 1JC,P = 39.9 Hz, i-CP), 135.2 (d, JC,P = 11.6 Hz, CHPh), 134.4 (d, JC,P = 11.1 Hz, CHPh), 131.8 (d, 4JC,P = 1.8 Hz, Cquart, p-CPh), 131.2 (d, 1JC,P = 42.2 Hz, i-CP), 130.6 (d, 4JC,P = 1.9 Hz, p-CPh), 130.2 (d, JC,P = 9.9 Hz, CHPh), 128.4 (d, JC,P = 10.0 Hz, CHPh), 105.7 (dd, 1JC,Rh = 12.3 Hz, 2JC,P = 6.8 Hz, dept(+), COD-CH), 71.0, (d, 1JC,Rh = 14.0 Hz, dept(+), COD-CH), 33.3 (d, 2JC,Rh = 2.2 Hz, dept(−), CODCH2), 29.1 (dept(−), COD-CH2). Additional resonances (s) for residually coordinated THF are observed at 68.1 and 25.7 ppm. 31 1 P{ H} NMR (CDCl3, 162 MHz): δ [ppm] = 30.9 (d, 1JP,Rh = 150.3 Hz, P-RhCl(cod)). IR (ATR): ν̃ [cm−1] = 3053 (vw), 2937 (vw), 2914 (vw), 2874 (w), 2829 (vw), 1597 (vw), 1570 (w), 1501 (m), 1481 (m), 1434 (m), 1393 (s), 1333 (w), 1307 (w), 1187 (w), 1157

(vw), 1143 (vw), 1092 (m), 1046 (w), 1028 (w), 1018 (w), 997 (w), 961 (w), 888 (w), 848 (m), 815 (w), 764 (m), 746 (m), 722 (vs), 694 (vs), 634 (w), 618 (w), 558 (w), 525 (vs), 504 (s), 487 (s), 454 (m), 421 (s). Raman (solid state): ν̃ [cm−1] = 3056 (vw), 2878 (vw), 2834 (vw), 2784 (vw), 1597 (s), 1498 (m), 1392 (vs), 1189 (w), 1088 (w), 1030 (vw), 1020 (vw), 1001 (w), 731 (w), 691 (vw), 634 (vw), 619 (vw), 566 (m), 504 (vw), 489 (vw), 420 (vw), 396 (w), 280 (m). Anal. Calcd (%) for [C108H104Cl4Rh4Mo2O8P4·(C4H8O)] (2471.36 g·mol−1): C 54.43, H 4.57; found C 54.36, H 4.75. Synthesis of [Mo2(p-O2CPhPPh2)4(IrCl(cod))4] (4). The phosphinefunctionalized metalloligand 1 (50.0 mg, 0.04 mmol, 1.00 equiv) and bis(1,5-cyclooctadiene)diiridium(I) dichloride (47.5 mg, 0.07 mmol, 2.00 equiv) were dissolved in THF (10 mL) and stirred at room temperature for 16 h. After a short while an orange solid precipitated. The mother liquor was decanted off, the product washed twice with additional THF (10 mL) and n-pentane (5 mL), and dried under vacuum. Crystalline yield: 36.1 mg (37%). 1 H NMR (CDCl3, 400 MHz): δ [ppm] = 8.33−8.19 (m, 8H, Ph), 7.78−7.65 (m, 24H, Ph), 7.46−7.35 (m, 24H, Ph), 5.26−5.13 (m, 8H, COD−CH), 2.78−2.68 (m, 8H, COD−CH), 2.30−2.11 (m, 16H, COD−CH2), 1.91−1.84 (m, 8H, COD−CH2), 1.65−1.52 (m, 8H, COD−CH 2). Additional resonances (m) for residually coordinated THF are observed at 3.73−3.65 and 1.87−1.81 ppm. 13 C{1H} NMR (CDCl3, 101 MHz): δ [ppm] = 176.5 (CO2R), 135.4 (d, JC,P = 11.2 Hz, CHPh), 134.6 (d, JC,P = 10.6 Hz, CHPh), 132.0 (Cquart, p-CPh), 130.8 (d, 4JC,P = 2.3 Hz, p-CPh) 130.6 (d, 1JC,P = 50.1 Hz, i-CP), 130.1 (d, JC,P = 10.6 Hz, CHPh), 128.4 (d, JC,P = 10.3 Hz, CHPh), 94.7 (d, 2JC,P = 14.3 Hz, dept(+), COD-CH), 53.9 (dept(+), COD-CH), 33.7 (d, 3JC,P = 2.8 Hz, dept(−), COD-CH2), 29.7 (dept(−), COD-CH2). One quaternary carbon resonance of i-CP is not detected, due to overlying signals; additional resonances (s) for residually coordinated THF are observed at 68.1 and 25.7 ppm. 31 1 P{ H} NMR (CDCl3, 162 MHz): δ [ppm] = 22.2 (s, P-IrCl(cod)). IR (ATR): ν̃ [cm−1] = 3054 (vw), 2935 (vw), 2914 (vw), 2878 (w), 2831 (w), 1598 (vw), 1570 (w), 1501 (s), 1483 (m), 1434 (m), 1395 (s), 1330 (w), 1307 (w), 1214 (vw), 1186 (w), 1157 (w), 1094 (s), 1046 (w), 1018 (m), 999 (m), 971 (w), 886 (m), 849 (m), 817 (w), 765 (m), 747 (m), 722 (vs), 695 (vs), 634 (w), 619 (w), 561 (m), 533 (s), 507 (s), 490 (s), 456 (s), 440 (s), 425 (s). Raman (solid state): ν̃ [cm−1] = 3057 (vw), 2913 (vw), 2884 (vw), 2834 (vw), 2785 (vw), 1598 (s), 1499 (m), 1401 (vs), 1393 (vs), 1191 (w), 1090 (w), 1029 (vw), 1020 (vw), 1001 (w), 732 (w), 693 (vw), 635 (vw), 620 (vw), 568 (m), 535 (vw), 505 (vw), 493 (vw), 425 (vw), 395 (m), 281 (m). Anal. Calcd (%) for [C108H104Cl4Ir4Mo2O8P4] (2756.50 g·mol−1): C 47.06, H 3.80; found C 46.90, H 3.97. Synthesis of [Mo2(p-O2CPhPPh2)4(BH3)4] (5). The phosphinefunctionalized metalloligand 1 (70.0 mg, 0.05 mmol, 1.00 equiv) was dissolved in THF (10 mL), and a borane−tetrahydrofuran solution (1.0 M in THF) (0.50 mL, 0.50 mmol, 10.0 equiv) was slowly added to the reaction mixture at ambient temperature, resulting in a red solution. The solution was stirred for 16 h, and after the solvent was concentrated under reduced pressure, n-pentane was added (15 mL) to precipitate a red solid. The residue was washed with additional npentane (10 mL) and dried under vacuum. Single crystals suitable for X-ray analysis were obtained from slow diffusion of n-pentane into a solution of 5 in THF. Yield (single crystals): 47.0 mg (65%). 1 H NMR (d8-THF, 400 MHz): δ [ppm] = 8.39−8.21 (m, 8H, Ph), 7.71−7.42 (m, 48H, Ph), 1.67−0.89 (br, 12H, BH3). Additional resonances (m) for residually coordinated THF are observed at 3.65− 3.59 and 1.81−1.74 ppm. 13C{1H} NMR (d8-THF, 101 MHz): δ [ppm] = 176.7 (d, 5JC,P = 1.0 Hz, CO2R), 135.1 (d, 1JC,P = 55.0 Hz, iCP), 134.3 (d, JC,P = 9.7 Hz, CHPh), 132.4 (d, 4JC,P = 2.2 Hz, p-CPh), 131.2 (d, JC,P = 10.2 Hz, CHPh), 130.5 (d, 1JC,P = 56.7 Hz, i-CP), 129.9 (d, JC,P = 10.1 Hz, CHPh). Additional carbon signals of CHPh are not detected, due to overlying signals at 134.3 ppm. 31P{1H} NMR (d8-THF, 162 MHz): δ [ppm] = 21.4 (P-BH3). 11B NMR (d8-THF, 128 MHz): δ [ppm] = −37.8 (br, BH3). IR (ATR): ν̃ [cm−1] = 3056 (vw), 2956 (vw), 2871 (vw), 2385 (m), 2345 (w), 1599 (vw), 1570 (w), 1495 (m), 1437 (m), 1396 (s), 1337 (w), 1308 (w), 1242 (vw), 1187 (w), 1160 (vw), 1131 (w), 1104 (m), 1057 (s), 1028 (w), 1018 9366

DOI: 10.1021/acs.inorgchem.8b01334 Inorg. Chem. 2018, 57, 9364−9375

Article

Inorganic Chemistry Scheme 2. Synthesis of Metalloligand 1 via Carboxylate Substitution from Molybdenum(II) Acetate with 4(Diphenylphosphino)benzoic Acid in THF

material.3,30,36 Typically, a direct conversion with carboxylic acids yields homoleptic compounds of the general formula [Mo2(O2CR)4], as in the case of Mo2(OAc)4.30 However, if the applied carboxylic acids bear additional functional groups, side products or undesired product formation may occur. Especially in case of phosphine-functionalized carboxylic acids a more suitable reaction pathway needs to be applied, as phosphine moieties are known to interact with molybdenum. Alternative methods use tetrakis(carboxylato)dimolybdenum(II) complexes or K4[Mo2Cl8] as precursors, in which the desired quadruple bond has already been formed.3,37,38 Especially the easily accessible tetraacetate Mo2(OAc)4 has proven to be a useful and versatile intermediate for preparing other, more challenging compounds, containing a Mo24+ unit.39,40 The successful application of 4-(diphenylphosphino)benzoic acid as a bifunctional ligand for heterometallic complex formation has already been addressed in literature for different metal combinations.34,41,42 The reaction of 4(diphenylphosphino)benzoic acid with K4[Mo2Cl8] was investigated by Kuang et al. and resulted in the formation of the phosphine-bearing compound [Mo2(p-O2CPh-PPh2)4] (1). However, the potential of this complex to act, due to its free phosphine moieties, as a fourfold metalloligand has only been investigated briefly.34 Interestingly, the high steric demand of the triphenylphosphine moieties prohibits the formation of [Mo2Cl4(PR3)4], as it is known for smaller phosphine ligands (R = Me, n-Pr, n-Bu),43,44 yet favors a bridged carboxylate paddlewheel structure. In an alternative pathway, which we report here, the phosphine-functionalized metalloligand 1 was obtained by a carboxylate exchange reaction in very good yields (83%; Scheme 2). Red single crystals of 1 were obtained by crystallization from a hot dimethyl sulfoxide solution, allowing characterization by X-ray diffraction. Compound 1 crystallizes in the triclinic space group P1̅ with one molecule in the asymmetric unit. Two solvent molecules of dimethyl sulfoxide coordinate axially to the free coordination sites of the dimolybdenum unit. The corresponding molecular structure in the solid state is shown in Figure 1, revealing the expected tetracarboxylatobridged paddlewheel scaffold. The molybdenum−molybdenum quadruple bond length is 2.1119(6) Å, which is a typical value for a quadruple Mo−Mo bond in a bidentate bridged coordination geometry (2.06−2.13 Å).45 All molybdenum− oxygen bond lengths to the carboxylic units are almost identical (Mo−O = 2.098−2.116 Å), indicating a highly symmetric arrangement in the solid state. The individual

(m), 999 (w), 884 (vw), 849 (m), 755 (m), 736 (vs), 722 (s), 692 (vs), 637 (m), 624 (m), 609 (m), 552 (w), 505 (s), 485 (s), 443 (m), 424 (s). Raman (solid state): ν̃ [cm−1] = 3060 (w), 2346 (vw), 1600 (vs), 1500 (s), 1407 (vs), 1188 (m), 1101 (w), 1030 (vw), 1020 (vw), 1001 (w), 728 (w), 637 (vw), 627 (vs), 611 (vw), 558 (m), 504 (vw), 491 (vw), 425 (vw), 402 (m), 279 (m). Anal. Calcd (%) for [C76H68B4Mo2O8P4] (1468.43 g·mol−1): C 62.16, 4.67; found C 60.98, H 4.87. Synthesis of [Mo2(p-O2CPhPPh2)4(AuCl)4] (6). In a first step 4(diphenylphosphino)benzoic acid (100 mg, 0.33 mmol, 1.00 equiv) and [AuCl(tht)] (105 mg, 0.33 mmol, 1.00 equiv) were dissolved in DCM (10 mL) and stirred at room temperature for 4 h. The solvent of the colorless solution was removed under vacuum, and the remaining solid was subsequently washed with n-pentane. The synthesized dimeric complex [(HO2CPhPPh2)(AuCl)]235 was in situ reacted with molybdenum(II) acetate (30.0 mg, 0.07 mmol, 0.21 equiv) in THF (10 mL). After it was stirred for 16 h, a dark red solid precipitated. The mother liquor was decanted off, and the solid was dried under vacuum. Single crystals suitable for X-ray analysis were obtained after slow cooling of a hot dimethyl sulfoxide solution. Yield (single crystals): 58.0 mg (35%). 1 H NMR (CDCl3, 400 MHz): δ [ppm] = 8.28−8.09 (m, 8H, Ph), 7.67−7.33 (m, 48H, Ph). An additional resonance (s) for residually coordinated DMSO is observed at 2.65 ppm. 13C{1H} NMR (CDCl3, 101 MHz): δ [ppm] = 134.4 (d, JC,P = 13.9 Hz, CHPh), 134.2 (d, JC,P = 14.0 Hz, CHPh), 132.6 (d, 4JC,P = 2.6 Hz, p-CPh), 130.7 (d, JC,P = 11.9 Hz, CHPh), 129.6 (d, JC,P = 12.0 Hz, CHPh), 128.0 (d, 1JC,P = 62.6 Hz, i-CP). Because of poor solubility, some quaternary carbon atoms (CO2R, i-CP) and CHPh cannot be detected. 31P{1H} NMR (CDCl3, 162 MHz): δ [ppm] = 33.2 (s, P-AuCl). IR (ATR): ν̃ [cm−1] = 1726 (vw), 1661 (vw), 1598 (w), 1587 (vw), 1570 (vw), 1500 (m), 1481 (m), 1435 (m), 1392 (s), 1307 (w), 1277 (vw), 1184 (w), 1160 (vw), 1142 (vw), 1099 (s), 1016 (m), 998 (m), 951 (vw), 916 (w), 848 (m), 764 (m), 747 (m), 725 (vs), 713 (s), 690 (vs), 632 (w), 617 (w), 570 (m), 557 (w), 535 (s), 505 (s), 491 (s), 464 (m), 427 (m). Raman (solid state): ν̃ [cm−1] = 3056 (vw), 1597 (s), 1496 (m), 1402 (vs), 1186 (m), 1095 (w), 1028 (vw), 1018 (vw), 1000 (w), 734 (w), 701 (vw), 633 (vw), 619 (vw), 576 (m), 538 (w), 494 (w), 429 (vw), 399 (w), 279 (m). Anal. Calcd (%) for [C76H56Au4Cl4Mo2O8P4· (CH3)2SO] (2420.89 g·mol−1): C 38.70, H 2.58, S 1.32; found C 38.67, H 2.96, S 0.88. X-ray Crystallographic Studies. Detailed XRD measurement description as well as crystal and structure refinement data are provided as Supporting Information. Additionally, selected bond lengths and angles of compounds 1, 2, 5, and 6 are given in the Supporting Information (Table S1 and Figures S1−S4).

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RESULTS AND DISCUSSION Syntheses and Characterization. A common pathway for the synthesis of dimolybdenum(II)-carboxylate paddlewheel complexes utilizes mononuclear Mo(CO)6 as starting 9367

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coordinated metal ions.30,46,47 For compound 1, the characteristic antisymmetric C−O stretching mode can be detected at ν̃a = 1493 cm−1, and the symmetric mode is detected at ν̃s = 1392 cm−1. This is in good agreement with data obtained for the structurally related dimolybdenum complex [Mo2(O2CPh)4] (ν̃a = 1494 cm−1, ν̃s = 1404 cm−1).30 Additional Raman spectra in the range from 500 to 200 cm−1 reveal a significant band at 399 cm−1, which is associated with the quadruple-bonded Mo−Mo stretching mode.48 This corresponds well with other bridged quadruple-bonded Mo24+ structures (typically in the range of 396−406 cm−1).47 Interestingly, if crystallization is induced by a slow diffusion of n-pentane into a THF solution of compound 1, a different modification (1′) is obtained. Compound 1′ crystallizes in the monoclinic space group P21/n with half of a molecule in the asymmetric unit. Compared to the solid-state structure of 1, obtained from dimethyl sulfoxide, no solvent molecules coordinating to the axial sites of the dimolybdenum unit are observed (Supporting Information, Figure S6). Instead, the phosphine moieties of the neighboring molecules seem to coordinate axially to both sides of the Mo2 unit, forming a coordination polymer (Figure 2). The distance between the phosphine moieties to the dimolybdenum unit is ∼3.5 Å, therefore no actual bond formation is assumed (insufficient data set). This coordination motif is not unexpected, as phosphines are known to coordinate axially to the free coordination sites of bridged Mo24+ complexes or even rearrange the structure by partial replacement of carboxylic units.49−51 An axial attachment of two PPh 3 ligands to [Mo 2 (O 2 CCF 3 ) 4 ] was investigated by Cotton in 1981, exhibiting Mo−P distances of 3.07(5) Å.46 The steric demand of the diphenylphosphinobenzoate ligands in 1 seems to prohibit such a short distance, and only weak axial interactions are realized, which seem to be easily disrupted by coordination of a strong donor solvent, for example, DMSO. Additionally, 31P{1H} NMR measurements in d 8-THF indicate no direct phosphor molybdenum interaction in solution. Analogous results were received in

Figure 1. Molecular structure of 1 in the solid state. Hydrogen atoms and noncoordinating solvent molecules (DMSO) are omitted for clarity.

carboxylate ligands are oriented in a nearly 90° angle to each other, as it is expected in a paddlewheel structure. In the 1H and 13C{1H} NMR spectra, a symmetric set of resonances is observed, indicating the successful formation of complex 1. All four phosphine moieties are chemically equivalent, showing a singlet resonance at δ = −5.18 ppm in the 31P{1H} NMR spectrum, which is in the usual range for triarylphosphines and is in agreement with the literature.34 Further information was obtained by vibrational spectroscopy. In a bridged coordination of a carboxylate unit (RCO2−), the two C−O bonds are equivalent, and the corresponding frequencies of the symmetric and antisymmetric stretching modes should be detectable as strong bands in the region of 1400−1600 cm−1, mainly depending on the ligand design and

Figure 2. Crystallization of metalloligand 1 from a THF solution leads to a coordination polymeric structure in the solid state (1′). Simplified cutout of 1′ is depicted. 9368

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Inorganic Chemistry Scheme 3. Synthesis of Heteromultimetallic Complexes (2−4 and 6) and the Borane Adduct 5

the noncoordinating solvent CDCl3, as only a single resonance for the noncoordinated phosphine moieties (δ = −4.57 ppm) is detected even at temperatures of 233 K (Supporting Information, Figure S59). This is in agreement with the observed dissociation of different phosphine adducts to [Mo 2 (O 2 CCF 3 ) 4 ] by 31 P{ 1 H} NMR spectroscopy (in CDCl3).51 The incorporation of phosphine functionalities in metalloligand 1 allows for the subsequent coordination of additional metal ions, preferably late transition metals (e.g., Au(I), Rh(I), and Ir(I)), thus enabling the formation of heteromultimetallic complexes with a dimolybdenum core unit. Furthermore, the choice of 4-(diphenylphosphino)benzoic acid as a bifunctional ligand should enable a conjugation between the different metal centers, inducing potential cooperative effects. To completely saturate the monodentate phosphine moieties, metalloligand 1 was reacted with 4 equiv of [AuC6F5(tht)] in dichloromethane, obtaining the heterometallic complex [Mo2(p-O2CPhPPh2)4(AuC6F5)4] (2) via ligand exchange (Scheme 3 a; Route A). Red single crystals were obtained from slow diffusion of npentane into a solution of 2 in THF. The hexametallic complex 2 crystallizes in the monoclinic space group C2/c with two molecules of THF coordinated axially to the Mo2 unit (Figure 3). The basic paddlewheel scaffold remains unchanged upon AuC6F5 complexation with a molybdenum−molybdenum quadruple bond length of 2.1082(14) Å. Regarding the Mo− Mo bond length, the influence of different axially coordinating solvent molecules must be considered, an effect that has been reported in literature.45 All four gold atoms are ligated in a linear fashion, as indicated by the P−Au−C6F5 bond angles of 179.0(3)° and 178.0(8)°. The composition of compound 2 was further confirmed by multinuclei NMR measurements (1H, 13C{1H}, 31P{1H} 19F{1H}) and by IR and Raman spectroscopies, as well as elemental analysis. In the 1H NMR spectrum a downfield shift of the aromatic resonances, compared to complex 1, is observed upon coordination of the AuC6F5 units. In the 31 1 P{ H} NMR spectrum a single resonance for 2, significantly downfield-shifted compared to metalloligand 1 (δ = −5.18 ppm), is detected at δ = 42.1 ppm in the splitting of a pseudoquintet (with a coupling constant J = 8.0 Hz), indicating a sole phosphine-gold coordination. The resonance

Figure 3. Molecular structure of 2 in the solid state. Hydrogen atoms and noncoordinating solvent molecules (THF) are omitted for clarity.

splitting is caused by a long-range phosphor coupling to the fluorine atoms of the C6F5 ligands, an effect that has already been observed in comparable complexes.52−54 In the 19F{1H} NMR spectrum, a characteristic set of three signals for the C6F5 moieties, with an integration ratio of 2:1:2, is observed at −116.5 ppm (o-F), −160.4 ppm (t, 3JF,F = 19.7 Hz, p-F), and −164.0 ppm (m-F), respectively. In the IR spectrum no significant shift of the symmetric and antisymmetric C−O stretching modes is detected (ν̃a = 1501 cm−1, ν̃s = 1393 cm−1), hence remaining rather unaffected upon gold(I) coordination. In the Raman spectrum a band, which can be assigned to the quadruple-bonded Mo−Mo stretching mode, is observed at ν̃ = 397 cm−1, which is similar to metalloligand 1 (ν̃ = 399 cm−1). However, the axial coordination of different solvent molecules needs to be considered.47 To obtain analogous multimetallic complexes with Rh(I) and Ir(I), metalloligand 1 was reacted with 2 equiv of [RhCl(cod)]2 and [IrCl(cod)]2, respectively. Upon phosphine coordination the dimeric Rh(I) and Ir(I) precursors dissociate 9369

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Figure 4. Molecular structures of 5 (left) and 6 (right) in the solid state. Hydrogen atoms and noncoordinating solvent molecules (THF and DMSO, respectively) are omitted for clarity.

2385 cm−1, supporting the successful formation of compound 5. To introduce yet another Au(I) source besides AuC6F5, metalloligand 1 was reacted with [AuCl(tht)]. However, a direct conversion with this Au(I) precursor has proven difficult, as decomposition was observed, most likely due to redox-induced side reactions. Nevertheless, utilizing the orthogonal properties of the bifunctional ligand 4(diphenylphosphino)benzoic acid, the versa reaction pathway was applied. A conversion of the known35 dimeric complex [pHO2CPh-PPh2-AuCl]2 with Mo2(OAc)4 leads to the formation of [Mo2(p-O2CPh-PPh2)4(AuCl)4] (6) (Scheme 3e; Route B), a heterometallic compound, which has been mentioned in literature before but was only scarcely characterized.34 Dark red single crystals of compound 6, suitable for X-ray analysis, were obtained by slow cooling of a hot DMSO solution. The hexametallic compound 6 crystallizes in the triclinic space group P1̅ with half of a molecule in the asymmetric unit and two solvent molecules axially coordinated to the dimolybdenum scaffold (Figure 4). The Mo−Mo quadruplebond length is 2.122(2) Å and therefore slightly longer than observed for the complexes 1, 2, and 5. All four gold(I) chloride units are ligated in a bent linear fashion, slightly deviating from linear 180°, as indicated by the P−Au−Cl bond angles of 173.2(2)° and 172.85(14)°. In the 31P{1H} NMR spectrum of compound 6, a single resonance at δ = 33.2 ppm is observed, indicating a complete and symmetrical coordination of the gold(I) chloride units to the phosphine moieties. Compared to the Au(I)-C 6 F 5 compound 2 (δ = 42.1 ppm) a significant upfield shift is detected. The characteristic antisymmetric and symmetric stretching modes of the carboxylate units in the IR spectrum (ν̃a = 1500 cm−1, ν̃s = 1392 cm−1) as well as the quadruplebonded Mo−Mo stretching mode in the corresponding Raman spectrum (ν̃ = 399 cm−1) are in the expected range. Steady-State Spectroscopy in Solution. The following two sections highlight the fine-tuning of the optical properties regarding the coordination of different metal complexes to the phosphine donor sites of metalloligand 1. The corresponding analysis correlates results from stationary absorption and fluorescence spectroscopies with pump−probe broadband transient absorption spectroscopy for complexes 1−5. Complex 6 was excluded, due to its bad solubility behavior in common organic solvents.

to obtain the heterometallic complexes [Mo2(p-O2CPhPPh2)4(RhCl(cod))4] (3) and [Mo2(p-O2CPhPPh2)4(IrCl(cod))4] (4) (Scheme 3b,c; Route A). Both compounds were recrystallized from a hot THF solution, yet no single crystals for X-ray analysis were obtained. Compounds 3 and 4 were investigated by NMR, IR, and Raman spectroscopies as well as elemental analysis. Because of a coupling to the NMR-active 103 Rh nuclei (s = 1/2), a sharp doublet resonance at δ = 30.9 ppm is observed (1JP,Rh = 150.3 Hz) in the 31P{1H} NMR spectrum of Rh(I) compound 3. The chemical shift as well as the coupling constant J are in good agreement with the literature-known compound [PPh3RhCl(cod)].55 Analogously, all phosphine moieties of Ir(I)-compound 4 are chemically equivalent, showing a singlet resonance at δ = 22.2 ppm. The IR and Raman spectra (ν̃(Mo−Mo) = 396 and 395 cm−1, respectively) of both compounds are almost identical, indicating, as expected, a similar structural formation of the two complexes. For subsequent analysis of the compounds’ photophysical properties, it seemed relevant to compare the heteromultimetallic complexes 2−4 with a related, yet metal-free structure. Thus, in a similar reaction, metalloligand 1 was reacted with an excess of BH3 (1.0 M THF solution) to yield the Lewis acid− base adduct [Mo2(p-O2CPhPPh2)4(BH3)4] (5) (Scheme 3d; Route A). Red single crystals of 5, suitable for X-ray analysis, were obtained by slow diffusion of n-pentane into a THF solution of 5. The compound crystallizes in the orthorhombic space group Pbcn with half of a molecule of 5 in the asymmetric unit (Figure 4). The Mo−Mo quadruple bond length of 2.1120(11) Å is in the same range as observed for the compounds 1 and 2. The phosphor boron bond distances are nearly alike with 1.908(9) Å and 1.915(9) Å. In the 31P{1H} NMR spectrum of compound 5 a single resonance at δ = 21.4 ppm is observed and relates to the borane-coordinated phosphine moieties. 11B NMR measurements correspondingly exhibit a broad signal for the BH3 moieties at δ = −37.8 ppm. The obtained NMR data are in good agreement with the simplified compound [BH3·PPh3].56 However, neither in the 11B nor in the 31P{1H} NMR spectrum a hyperfine structure is observed, most likely due to a nonsymmetric behavior of the complex in solution. Additionally, besides the symmetric and antisymmetric C−O stretching modes (ν̃a = 1495 cm−1, ν̃s = 1396 cm−1), the characteristic B− H stretching mode57 is detected in the IR spectrum at ν̃ = 9370

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redshift of the MLCT band with a maximum absorption at 475 nm. The AuC6F5-coordination in complex 2 results in the highest bathochromic shift of the MLCT absorption (peak at 486 nm) band at ∼883 cm−1. Furthermore, conjugation of the π-acceptor-ligands could be slightly increased by the fluorinated phenyl rings attached to the Au(I) ions, leading to the highest decrease in energy of the relevant orbitals.17 MLCT absorptions of 3 and 4 show comparable redshifts to 477 and 478 nm. The heteromultimetallic compounds 3 and 4 attract attention by additional shoulders on the high-energy side of the MLCT band, which is tentatively traced back to structural changes induced by the sterically demanding RhCl(cod)/ IrCl(cod) entities thatin contrast to AuC6F5 or the BH3 groupmay lead to additional transitions by modified orbital coupling. Molar extinction coefficients for 1, 2, and 5 were readily determined being in the order of a few 1 × 104 M−1· cm−1, which is in good agreement with other comparable group 6 metal complexes (Supporting Information, Figure S12).20,61 In contrast, quantitative values were not obtained for the Rh(I) and Ir(I) complexes 3 and 4, due to limited solubility. Fluorescence spectra of complexes 1−5 in THF were measured at room temperature after excitation at 480 nm. The corresponding emission spectra range between 500 and 850 nm (Figure 5). Phosphorescence is also expected for the quadruply bonded Mo scaffold,17,20 but it is beyond the available detection range of our fluorescence spectrometer. All Mo compounds (1−5) showed a spectrally broad fluorescence band between 500 and 800 nm with a comparable band shape but slightly shifted emission maxima (see Table 1). For reference compound 1, the fluorescence maximum was observed at 612 nm, which corresponds to a Stokes shift of 5120 cm−1 (see Supporting Information, Table S2). According to their UV−vis absorption spectra, the emission bands of 2−5 are slightly red-shifted (see Table 1) but with somewhat lower Stokes shifts. In line with the bathochromic shift of the absorption spectrum, the fluorescence spectrum of 2 peaks again at the highest red-shifted wavelength (639 nm). For compounds 3−5, there are small variations of the redshifts when compared with the absorption spectra. Fluorescence quantum yields (QYs) were determined by using the calibration standard Rh6G in EtOH after excitation at 480 nm. All complexes investigated (1−5) exhibit a rather low QY, which is in the range of (0.3−0.5) × 10−3 (see Table 1). This is in good agreement with QYs observed for comparable dimolybdenum paddlewheel complexes.20 Further inspection reveals that the heteromultimetallic compounds 2−4 show slightly increased QYs (especially 3) compared to reference system 1 and complex 5. To further substantiate this trend, transient absorption measurements were performed, which will be reported in the next section. Transient Absorption Spectroscopy in Solution. Complexes 1−5 were investigated by femtosecond TA spectroscopy in THF solution. The excitation wavelength was set to 515 nm on the low-energy tail of MLCT absorption bands to reduce excess energy in the excited state. Because of pump−pulse scattering the spectral range of roughly 20 nm (10 nm to each side of the peak at 515 nm) in the vicinity of the excitation wavelength (gray shaded area in the TA spectra) was not considered for analysis in all TA spectra (350 to 700 nm). TA spectra of the AuC6F5-coordinated compound 2 is shown in Figure 6 at specific delay times. Results of reference

UV−vis absorption spectra of dimolybdenum compounds 1−5 were recorded in THF solution at room temperature (Figure 5). All Mo complexes show a pronounced absorption

Figure 5. UV−Vis absorption spectra of complexes 1 (black), 2 (yellow), 3 (green), 4 (blue), and 5 (red) in THF (deoxygenized and water-free) and corresponding emission spectra after excitation at 480 nm and detection between 500 and 850 nm. Fluorescence intensity is given relative to optical density (OD) = 0.1 of the respective sample solution. Slit width was set to 5 nm for excitation as well as for detection.

band between 400 and 550 nm due to a strong MLCT. Moreover, there are dominant absorption transitions in the UV region (