Synthesis, Structures, and Reactions of Manganese Complexes

Sep 20, 2010 - Kevin D. Welch,† William G. Dougherty,‡ W. Scott Kassel,‡ Daniel L. .... Bullock, R. M.; Rakowski DuBois, M.; DuBois, D. L. Energ...
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Organometallics 2010, 29, 4532–4540 DOI: 10.1021/om100668e

Synthesis, Structures, and Reactions of Manganese Complexes Containing Diphosphine Ligands with Pendant Amines Kevin D. Welch,† William G. Dougherty,‡ W. Scott Kassel,‡ Daniel L. DuBois,† and R. Morris Bullock*,† †

Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352, and ‡Department of Chemistry and Biochemistry, Villanova University, Villanova, Pennsylvania 19085 Received July 8, 2010

Addition of the pendant amine ligand PNRP (PNRP = Ph2PCH2NRCH2PPh2; R = Me, Ph, n-Bu) to Mn(CO)5Br gives fac-Mn(PNRP)(CO)3Br. Photolysis of fac-Mn(PNRP)(CO)3Br with dppm [dppm = 1,2-bis(diphenylphosphino)methane] provides mixed bis(diphosphine) complexes, transMn(PNRP)(dppm)(CO)(Br). Reaction of trans-Mn(PNRP)(dppm)(CO)(Br) with LiAlH4 leads to trans-Mn(PNRP)(dppm)(CO)(H), which has been characterized by crystallography. Mn(P2PhN2Bn)(dppm)(CO)(H) [P2PhN2Bn = 1,5-diphenyl-3,7-dibenzyl-1,5-diaza-3,7-diphosphacyclooctane] can be prepared in a similar manner; its structure has one chelate ring in a chair conformation and the second in a boat conformation. The boat-conformer ring directs the nitrogen of the ring toward the carbonyl ligand, and the N 3 3 3 C distance between one N of the P2PhN2Bn ligand and CO is 3.171(4) A˚, indicating a weak interaction between the N of the pendant amine and the CO ligand. Reaction of NaBArF4 (ArF = 3,5-bis(trifluoromethyl)phenyl) with Mn(P-P)(dppm)(CO)(Br) produces the cations [Mn(P-P)(dppm)(CO)]þ. The crystal structure of [Mn(PNMeP)(dppm)(CO)][BArF4] shows two very weak agostic interactions between C-H bonds on the phenyl ring and the Mn. The cationic complexes [Mn(P-P)(dppm)(CO)]þ react with H2 to form dihydrogen complexes [Mn(H2)(P-P)(dppm)(CO)]þ (Keq = 1-90 atm-1 in fluorobenzene, for a series of different P-P ligands). Similar equilibria with N2 produce [Mn(N2)(P-P)(dppm)(CO)]þ (Keq generally 1-3.5 atm-1 in fluorobenzene).

Introduction Incorporation of a pendant amine functionality1 into a diphosphine ligand can cause large changes in reactivity. The pendant amines are generally intended to function as proton relays, and they are designed to be close enough to interact with H2 or H ligands on the metal, but not close enough to form direct M-N bonds. While the initial intent of incorporation of pendant amines was to facilitate intramolecular and intermolecular proton transfer reactions, it has become clear that pendant amines also provide additional advantages, including stabilization of binding of H2 or CO ligands to a metal, lowering the barrier for heterolytic cleavage of H2, facilitating proton-coupled electron transfer reactions, and lowering overpotentials in electrocatalytic reactions. Incorporation of pendant amines into the diphosphine ligand of [Ni(diphosphine)2]2þ complexes led to a decrease of about 0.7 V in the overpotential for oxidation of H2 to *To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Rakowski DuBois, M.; DuBois, D. L. Acc. Chem. Res. 2009, 42, 1974–1982. (b) Rakowski DuBois, M.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62–72. (c) Rakowski DuBois, M.; DuBois, D. L. C. R. Chim. 2008, 11, 805–817. (2) Curtis, C. J.; Miedaner, A.; Ciancanelli, R.; Ellis, W. W.; Noll, B. C.; DuBois, M. R.; DuBois, D. L. Inorg. Chem. 2003, 42, 216–227. pubs.acs.org/Organometallics

Published on Web 09/20/2010

protons.2 The presence of two positioned proton relays3 was found to lead to faster catalytic rates in Ni electrocatalysts for oxidation of H2 compared to catalysts with just one positioned proton relay.4 Cobalt diphosphine complexes with a pendant amine are electrocatalysts for production of H2 by reduction of protons,5,6 but related catalysts lacking the pendant amine are essentially inactive.5 Iron complexes with diphosphines containing pendant amines have also been studied;7-9 rapid intramolecular proton/hydride exchange (3) Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.; DuBois, M. R.; DuBois, D. L. J. Am. Chem. Soc. 2006, 128, 358–366. (4) Yang, J. Y.; Bullock, R. M.; Shaw, W. J.; Twamley, B.; Fraze, K.; Rakowski DuBois, M.; DuBois, D. L. J. Am. Chem. Soc. 2009, 131, 5935–5945. (5) Jacobsen, G. M.; Yang, J. Y.; Twamley, B.; Wilson, A. D.; Bullock, R. M.; Rakowski DuBois, M.; DuBois, D. L. Energy Environ. Sci. 2008, 1, 167–174. (6) Wiedner, E. S.; Yang, J. Y.; Dougherty, W. G.; Kassel, W. S.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L., Organometallics 2010, ASAP; doi: 10.1021/om100395r. (7) Henry, R. M.; Shoemaker, R. K.; Newell, R. H.; Jacobsen, G. M.; DuBois, D. L.; Rakowski DuBois, M. Organometallics 2005, 24, 2481– 2491. (8) Henry, R. M.; Shoemaker, R. K.; DuBois, D. L.; DuBois, M. R. J. Am. Chem. Soc. 2006, 128, 3002–3010. (9) (a) Jacobsen, G. M.; Shoemaker, R. K.; Rakowski DuBois, M.; DuBois, D. L. Organometallics 2007, 26, 4964–4971. (b) Jacobsen, G. M.; Shoemaker, R. K.; McNevin, M. J.; Rakowski DuBois, M.; DuBois, D. L. Organometallics 2007, 26, 5003–5009. r 2010 American Chemical Society

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can be observed8 between Fe-H and N-H when structural and thermochemical properties are appropriately matched. We have prepared nickel complexes with two different diphosphines, but in some cases, these nickel complexes are difficult to obtain in pure form, since ligand redistribution reactions occur, converting [Ni(P-P)(P0 -P0 )]2þ into equilibrium mixtures that contain significant amounts of [Ni(P-P)2]2þ and [Ni(P0 -P0 )2]2þ.4 In an expansion of our studies that have previously concentrated on Ni, Co, and Fe complexes, we have begun an investigation of Mn complexes with two diphosphine ligands. We report here our initial studies of the synthesis, structures, and reactivity of manganese complexes that contain a diphosphine ligand with pendant amines. The synthetic procedure reported here provides a route to complexes containing two different diphosphine ligands. This allows the incorporation of two diphosphines that have different electronic and steric properties.

Results Syntheses and Characterization of Mixed Diphosphine Bromide Complexes. Addition of an excess of the pendant amine ligand PNMeP (PNMeP = Ph2PCH2N(Me)CH2PPh2) to Mn(CO)5Br in toluene results in the loss of two CO ligands and addition of a single equivalent of the diphosphine, yielding fac-Mn(PNMeP)(CO)3Br, as a yellow solid (eq 1). Even with two equivalents of diphosphine ligand added, only one diphosphine was incorporated into the product. The 31 P NMR spectrum exhibits a singlet at δ 20.9, and three carbonyl stretching bands are observed in the IR spectrum at 2026, 1957, and 1924 cm-1, both consistent with a facial geometry. The symmetric complex trans-Mn(dppm)2(CO)(Br)

[dppm = 1,2-bis(diphenylphosphino)methane] was prepared by Reimann and Singleton by photolysis of Mn(CO)5Br with dppm.10 We found that photolysis of a toluene solution of Mn(PNMeP)(CO)3Br in the presence of dppm results in the formation of the mixed bis(diphosphine) complex trans-Mn(PNMeP)(dppm)(CO)(Br); eq 1. This compound can be isolated as an orange solid. Infrared spectroscopy of Mn(PNMeP)(dppm)(CO)(Br) in CH2Cl2 shows a single carbonyl absorption at 1842 cm-1, consistent with the expected monocarbonyl species. A 31P NMR spectrum shows two distinct, broad resonances at δ 45.5 and 15.7 (line widths of 302 and 220 Hz, respectively) with no discernible coupling. The broad resonances observed in this and other Mn complexes in the 31P and 1H NMR spectra are due to the quadrupolar 55Mn nucleus (100% abundant, I = 5/2). The complexes trans-Mn(PNPhP)(dppm)(CO)(Br), trans-Mn(PNnBuP)(dppm)(CO)(Br), trans-Mn(dppp)(dppm)(CO)(Br) (10) Reimann, R. H.; Singleton, E. J. Organomet. Chem. 1972, 38, 113–119.

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Figure 1. Structure of Mn(PNMeP)(dppm)(CO)(Br) (30% probability ellipsoids). Only the ipso carbons of the Ph groups are shown.

[dppp = 1,2-bis(diphenylphosphino)propane], and transMn(P2PhN2Bn)(dppm)(CO)(Br) (eq 2) were prepared and characterized in a similar fashion using the appropriate diphosphine in place of PNMeP.

The structure of Mn(PNMeP)(dppm)(CO)(Br) was determined by single-crystal X-ray diffraction. Figure 1 shows the molecular structure, and Table 1 lists bond distances and angles for Mn(PNMeP)(dppm)(CO)(Br) and three other complexes to be discussed later. Syntheses and Characterization of Manganese Hydride Complexes Mn(P-P)(dppm)(CO)(H). The reaction of the manganese bromide complex Mn(PNMeP)(dppm)(CO)(Br) with LiAlH4 in THF, followed by treatment with water, results in the formation of the manganese hydride complex trans-Mn(PNMeP)(dppm)(CO)(H), which was isolated as a

bright yellow solid. Both 31P and 1H NMR spectra are consistent with a trans geometry for the diphosphine ligands, as observed in the bromide. The hydride resonance for Mn(PNMeP)(dppm)(CO)(H) is observed in the 1H NMR spectrum at δ -3.58 as a triplet of triplets of doublets. The two larger couplings are due to coupling of the hydride to phosphorus (2JPH = 49.3, 34.2 Hz). A 31P-decoupled 1H NMR spectrum simplifies the hydride signal to a doublet with a

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Table 1. Bond Distances and Bond Angles for Mn(PNMeP)(dppm)(CO)(Br), Mn(PNMeP)(dppm)(CO)(H), Mn(P2PhN2Bn)(dppm)(CO)(H), and [Mn(PNMeP)(dppm)(CO)]þ Mn(PNMeP) (dppm)(CO)(Br)

Mn(P2PhN2Bn) (dppm)(CO)(H)

[Mn(PNMeP) (dppm)(CO)]þ

2.2220(7) 2.2333(8) 2.2769(7) 2.2618(7)

2.1835(8) 2.1980(8) 2.2263(9) 2.2394(9)

2.2665(7) 2.2478(8) 2.2945(7) 2.3204(5)

1.63(3) 1.783(3) 1.170(3)

1.63(4) 1.782(3) 1.179(3)

1.744(3) 1.169(3)

89.01(3) 99.71(3) 166.16(3) 165.13(3) 97.99(3) 71.09(2)

80.51(3) 99.30(3) 163.18(3) 155.96(3) 100.26(3) 73.09(3)

88.70(3) 101.77(3) 172.54(3) 167.51(3) 98.50(2) 70.85(2)

105(1) 107(1) 59(1) 61(1) 87.78(8) 93.65(8) 98.67(8) 103.58(8)

88(1) 82(1) 74(1) 75(1) 94.19(9) 100.49(9) 103.50(9) 102.13(9)

91.46(8) 92.88(8) 93.63(8) 90.15(8)

155(1) 178.4(2)

176(1) 176.3(2)

179.0(2)

Mn(PNMeP) (dppm)(CO)(H) Bond Distances, A˚

Mn-P1(5) Mn-P2(6) Mn-P3(8) Mn-P4(7) Mn-Br Mn-H Mn-CO C-O

2.2892(6) 2.2816(8) 2.3264(8) 2.3322(6) 2.5770(5) 1.777(3) 1.084(3)

Bond Angles, deg P1-Mn-P2 P1-Mn-P3 P1-Mn-P4 P2-Mn-P3 P2-Mn-P4 P3-Mn-P4 P1-Mn-Br P2-Mn-Br P3-Mn-Br P4-Mn-Br P1-Mn-H P2-Mn-H P3-Mn-H P4-Mn-H P1-Mn-CO P2-Mn-CO P3-Mn-CO P4-Mn-CO Br-Mn-CO H-Mn-CO Mn-C-O

89.54(3) 100.30(3) 170.27(3) 168.92(3) 99.15(3) 70.67(2) 85.35(2) 84.01(2) 101.73(2) 99.82(2)

89.96(8) 90.30(8) 84.67(8) 85.66(8) 172.64(8) 179.6(2)

Figure 3. Structure of Mn(PNMeP)(dppm)(CO)(H) (30% probability ellipsoids). Only the ipso carbons of the Ph groups are shown.

Figure 2. Spectra of Mn(PNMeP)(dppm)(CO)(H) in THF-d8 showing the (A) hydride and (B) methylene resonances in the 1 H NMR (bottom) and with broadband 31P decoupling (top). The peak at δ 3.58 is the residual proton signal from THF-d8.

coupling of 7.5 Hz. The same coupling constant is observed for the resonance at δ 3.71, which has been assigned as one of the protons on the CH2 group of the dppm ligand. Thus a

four-bond coupling between the Mn-H and one C-H on the dppm ligand is observed (4JHH = 7.5 Hz). This is an unusually large four-bond coupling, but the iron hydride trans-[HFe(PEtNMePEt)(dmpm)(MeCN)]þ (PEtNMePEt = Et2PCH2NMeCH2PEt2; dmpm = Me2PCH2PMe2) exhibits a 4JHH = 4 Hz coupling between the Fe-H and one C-H on the dmpm ligand, and 4JHH = 3.5 Hz was found for [HFe(PEtNMePEt)(dmpm)(CO)]þ.7 Vapor diffusion of hexane into a THF solution of Mn(PNMeP)(dppm)(CO)(H) produced single crystals suitable for X-ray diffraction. A drawing of the structure is presented in Figure 3, and selected bond distances and angles are given in Table 1. Of particular note is the significant distortion of the hydride ligand from idealized octahedral geometry. The

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hydride is bent toward one C-H (H40A in Figure 3) of the methylene group of the dppm ligand, with a H-Mn-CO angle of 155(1)°. The distance between the hydride and the closest H of the CH2 group is 2.10(3) A˚. The structure of Mn(PNMeP)(dppm)(CO)(H) was determined by X-ray diffraction, so the distances and angles involving hydrogen are subject to much more uncertainty than those determined by neutron diffraction. The distortions due to this interaction in Mn(PNMeP)(dppm)(CO)(H) could be less pronounced than indicated in this X-ray structure; attempts to determine the structure by neutron diffraction are underway. This C-H 3 3 3 H-Mn distance of 2.10(3) A˚ between the C-H on an alkyl group and a Mn-H bond is very similar to the C-H 3 3 3 H-Mn distance of 2.101(3) A˚ determined by neutron diffraction for the interaction of the aromatic C-H bond of a phenyl group with the Mn-H bond in cis-HMn(CO)4PPh3.11 The C-H 3 3 3 H-Mn interaction in HMn(CO)4PPh3 is due to a protic-hydridic hydrogen-bonding

interaction. An experimental charge density determination of HMn(CO)4PPh3 showed an effective atomic charge of -0.4e on the manganese hydride and a þ0.3e on the C-H of the phenyl group. The electrostatic energy of this Hδþ 3 3 3 Hδ- interaction was calculated to be 5.7 kcal/mol. Crabtree and co-workers have reported extensive studies12 on related interactions involving N-H 3 3 3 H-Ir interactions; their experimental and computational studies resulted in a range of 5.7-7.1 kcal/mol for the estimated hydrogen bond strength.13 Related complexes with N-H 3 3 3 H-Ir interactions were reported by Morris and co-workers.14 These unconventional H 3 3 3 H interactions have been referred to as “dihydrogen bonds”.15 The hydride Mn(P2PhN2Bn)(dppm)(CO)(H) was prepared from Mn(P2PhN2Bn)(dppm)(CO)(Br) by reaction with LiAlH4 in an analogous fashion to Mn(PNMeP)(dppm)(CO)(H). This compound has a hydride resonance in the 1 H NMR at δ -5.11. Like its PNMeP analogue, it is split into a triplet of triplets of doublets (J = 46, 38, 7.3 Hz) in which the larger couplings correspond to phosphorus coupling and the smaller coupling matches the coupling to one of the C-H groups on the dppm ligand. The structure of Mn(P2PhN2Bn)(dppm)(CO)(H) was determined by X-ray diffraction of single crystals grown through the layering of hexane on a methylene chloride solution of the hydride. The cyclic P2PhN2Bn ligand orients with one chelate ring in a chair conformation and the second (11) Abramov, Y. A.; Brammer, L.; Klooster, W.; Bullock, R. M. Inorg. Chem. 1998, 37, 6317–6328. (12) Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle, T. F. Acc. Chem. Res. 1996, 29, 348–354. (13) Peris, E.; Lee, J. C., Jr.; Rambo, J. R.; Eisenstein, O.; Crabtree, R. H. J. Am. Chem. Soc. 1995, 117, 3485–3491. (14) (a) Park, S.; Ramachandran, R.; Lough, A. J.; Morris, R. H. J. Chem. Soc., Chem. Commun. 1994, 2201–2202. (b) Lough, A. J.; Park, S.; Ramachandran, R.; Morris, R. H. J. Am. Chem. Soc. 1994, 116, 8356– 8357. (15) (a) Richardson, T.; de Gala, S.; Crabtree, R. H.; Siegbahn, P. E. M. J. Am. Chem. Soc. 1995, 117, 12875–12876. (b) Custelcean, R.; Jackson, J. E. Chem. Rev. 2001, 101, 1963–1980.

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Figure 4. Structure of Mn(P2PhN2Bn)(dppm)(CO)(H) (30% probability ellipsoids). Only the ipso carbons of the Ph groups are shown.

in a boat conformation. The boat-conformer ring directs the nitrogen of the ring toward the carbonyl ligand. The N 3 3 3 C distance between one N of the P2PhN2Bn ligand and CO is 3.171(4) A˚. The carbonyl ligand displays a Mn-C-O angle of 176.3(2)° with the oxygen tilting away from nitrogen. The hydride shows a distortion from octahedral geometry in a fashion similar to that seen in the solid state for Mn(PNMeP)(dppm)(CO)(H). A H-Mn-CO angle of 176.7(14)° directs the hydride toward the dppm methylene group. The distance between the hydride and the closest C-H is 2.76(4) A˚, so the distortions are significantly less pronounced in Mn(P2PhN2Bn)(dppm)(CO)(H) compared to those found in Mn(PNMeP)(dppm)(CO)(H). The interaction of the P2PhN2Bn ligand with the CO in Mn(P2PhN2Bn)(dppm)(CO)(H) is similar to that observed previously in the crystal structure of [Ni(CO)(P2CyN2Bn)2]2þ (Cy = cyclohexyl).16 The interaction of the P2CyN2Bn ligand with the CO ligand on Ni is sufficiently strong to cause binding of the CO to Ni, since Ni complexes such as [Ni(dppe)2]2þ, with diphosphines that have no pendant amine, do not bind CO. The crystal structure of [Ni(CO)(P2CyN2Bn)2]2þ showed that the Ni-C-O angle was slightly nonlinear (177.5(4)°). In this Ni complex with two P2CyN2Bn ligands, the N 3 3 3 C distances were 3.30 and 3.38 A˚, which is longer than the N 3 3 3 C distance observed in Mn(P2PhN2Bn)(dppm)(CO)(H). For comparison to our Mn complexes with diphosphines containing pendant amines, the manganese hydride Mn(dppe)2(CO)(H) [dppe = 1,2-bis(diphenylphosphino)ethane] was synthesized from Mn(dppe)2(CO)(Br) through an analogous reaction with LiAlH4. A hydride resonance is observed at δ -7.32 as a quintet (JPH = 44 Hz) in the 1H NMR spectrum. Unlike hydrides Mn(PNMeP)(dppm)(CO)(H) and Mn(P2PhN2Bn)(dppm)(CO)(H), the coupling observed on the hydride is exclusively due to P-H coupling, as evidenced by this resonance collapsing to a singlet in the 31P-decoupled 1H NMR. Formation of Mn Cations [Mn(P-P)(dppm)(CO)]þ. The reaction of 1.1 equivalents of NaBArF4 (ArF = 3,5-bis(trifluoromethyl)phenyl) with the mixed diphosphine bromide complex Mn(PNMeP)(dppm)(CO)(Br) in fluorobenzene results in the formation of a deep blue solution. The reaction mixture shows two singlets in the 31P NMR spectrum at δ 52.4 and 30.3 (with line widths of 315 and 90 Hz, respectively). (16) Wilson, A. D.; Fraze, K.; Twamley, B.; Miller, S. M.; DuBois, D. L.; Rakowski DuBois, M. J. Am. Chem. Soc. 2008, 130, 1061–1068.

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A single CO stretching band is observed at 1843 cm-1. The characterization of this complex as the cationic [Mn(PNMeP)(dppm)(CO)]þ is further supported by its reactivity under H2 or N2 (vide infra). The other bromides Mn(P-P)(dppm)(CO)(Br) show analogous reactivity with NaBArF4, producing deep blue solutions with similar spectroscopic

Figure 5. Structure of [Mn(PNMeP)(dppm)(CO)]þ (30% probability ellipsoids).

characteristics. Kubas and co-workers17,18 prepared similar complexes [Mn(dppe)2(CO)]þ[BArF4]- and [Mn(depe)2(CO)]þ[Ga(C 6 F 5 )4 ]- [depe = 1,2-bis(diethylphosphino)ethane] by analogous reactions, and their complexes were also deep blue. These complexes appear to be 16-electron complexes, but they have one or more weak agostic interactions between a C-H bond on the ligand and the Mn. These cationic complexes are extremely sensitive to air and solvent impurities and are most easily studied by in situ generation by vacuum transfer of fluorobenzene onto the reactants in an NMR tube. Attempts to isolate the BArF4 salts of these complexes repeatedly resulted in degradation. Addition of hexane or pentane or prolonged exposure to the argon atmosphere of the glovebox resulted in a visible change of the color of the solution from deep blue to green and the formation of multiple unidentified products in the 31 P NMR spectrum. Evaporation of solvent from the reaction mixture also resulted in a color change from blue to green of the reaction solution and formation of unidentified products. Dark blue single crystals of [Mn(PNMeP)(dppm)(CO)][BArF4] were obtained as a mixture with amorphous solids from diffusion of hexane into a fluorobenzene reaction solution. The X-ray structure is presented in Figure 5, and selected bond lengths and angles are given in Table 1. The unit cell of the crystal shows two unique structures with very similar structural features. Each structure is square pyramidal with an open coordination site trans to the carbonyl ligand; one of the two structures is shown in Figure 5. Two ortho-hydrogens from phenyl groups of opposite phosphines are aligned toward the open coordination site, creating two agostic interactions. The Mn 3 3 3 H distances are 2.7562(3) and 3.1212(4) A˚, indicating very weak agostic interactions. A similar structure with two agostic interactions from C-H bonds of a Ph ring was previously found by Kubas and co-workers for [Mn(dppe)2(CO)]þ.17 The Mn 3 3 3 H distances for [Mn(dppe)2(CO)]þ were found to be 2.89(6) and 2.98(6) A˚, indicating weak agostic interactions. A structural study of the related complex with Et groups on the diphosphine ligand, (17) King, W. A.; Luo, X.-L.; Scott, B. L.; Kubas, G. J.; Zilm, K. W. J. Am. Chem. Soc. 1996, 118, 6782–6783. (18) King, W. A.; Scott, B. L.; Eckert, J.; Kubas, G. J. Inorg. Chem. 1999, 38, 1069–1084.

[Mn(depe)2(CO)]þ [depe = 1,2-bis(diethylphosphino)ethane], did not locate the H atoms on the ethyl groups, but idealized positions for them were calculated as Mn 3 3 3 H distances of 3.30 and 3.42 A˚. These long distances were interpreted as being “suggestive of van der Waals contacts with perhaps only a small contribution from agostic bonding”.18 In contrast to the deep blue solutions obtained for all the cationic Mn complexes discussed above, the reaction of Mn(P2PhN2Bn)(dppm)(CO)(Br) with NaBArF4 in fluorobenzene produces a red solution. A 31P NMR spectrum of the reaction shows two singlets, one very broad, at δ 24.6 and 23.4 (line widths of 262 and 76 Hz, respectively), consistent with retention of a plane of symmetry in the molecule. The relatively upfield shift of the phosphorus signals along with the pronounced difference in color suggests a different structure and bonding relative to the other cationic complexes discussed above. However, the CO stretching frequency of 1843 cm-1 is nearly identical to those found in the other Mn cationic complexes. Initial attempts at isolation of this complex have been unsuccessful, but further attempts are continuing. Formation of Manganese Complexes with H2 and N2 Ligands. The addition of 1 atm of H2 to the deep blue solutions of the Mn cations [Mn(P-P)(dppm)(CO)]þ described above results in a color change to yellow rapidly upon mixing. Removal of the H2 atmosphere by three freeze-pump-thaw cycles of the solution returns the original blue color of the Mn cation. 31 P NMR spectra of these yellow solutions under H2 show a pair of new singlets, slightly shifted from those seen in [Mn(P-P)(dppm)(CO)]þ (see Table 2 for spectroscopic details). The 1H NMR spectra contain one broad singlet resonance for each complex between δ -2.9 and -3.7 (with line widths ranging from 44 to 97 Hz), consistent with the presence of a dihydrogen ligand. To determine hydrogen binding constants for these complexes, each solution was allowed to equilibrate under 100 Torr of H2 gas. 31P NMR spectra of the resulting green solutions show a mixture of dihydrogen and [Mn(P-P)(dppm)(CO)]þ complexes, which was used to calculate the binding constant, Keq(H2). Exposure of each of the blue cation solutions to an atmosphere of N2 results in the formation of a green solution that shows resonances consistent with a mixture of [Mn(P-P)-

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Table 2. Spectroscopic Data for Cationic Manganese Complexes and Equilibrium Constants for Binding of H2 or N2a [Mn(P-P) (P0 -P0 )(CO)]þ Mn cation [Mn(dppe)2(CO)]þ [Mn(PNMeP)(dppm)(CO)]þ [Mn(PNPhP)(dppm)(CO)]þ [Mn(PNnBuP)(dppm)(CO)]þ [Mn(dppp)(dppm)(CO)]þ [Mn(P2PhN2Bn)(dppm)(CO)]þ

31

P (δ)

83.8 52.4, 30.3 54.5, 30.3 53.5, 30.7 53.4, 29.4 24.6, 23.4

IR (cm-1) 1843 1841 1842 1840 1839 1843

H2 complex 1

31

H (δ)b

-7.2 (55) -3.7 (55) -3.4 (44) -3.5 (97) -2.9 (69) -3.2 (56)

P (δ)

89.5 50.5, 31.9 51.4, 31.1 50.7, 31.5 52.7, 30.6 55.4, 37.4

N2 complex Keq(H2) (atm-1) 56 26 32 25 90 1.1

31

P (δ)

75.3 40.7, 21.6 46.2, 22.6 46.4, 22.2 44.0, 22.1 42.4, 31.0

Keq(N2)c (atm-1) 1.4 (58) 2.8 (74) 3.5 (78) 2.4 (71) 1.1 (52) 2σ(I)]a R indices (all data)a

P)(dppm)(CO)(Br) 3 Mn(PNMeP)(dppm)(CO)(H) 3 Mn(P2PhPN2Bn)(dppm)(CO)(H) 3 2 [Mn(PNMeP)(dppm)(CO)] 3 1.5(C6H5F) THF 2(CH2Cl2) BArF4

C62H56.50BrF1.50MnNOP4 1118.81 100(2) triclinic P1 12.8796(5) 13.5776(5) 17.9331(10) 102.992(2) 102.014(2) 112.728(2) 2662.3(2) 2 1.167 0.71073 1.396 0.16  0.09  0.08 1.23 to 33.34 42 297 19 930 [R(int) = 0.0307]

C57H58MnNO2P4 967.86 100(2) monoclinic C2/c 36.1746(7) 13.3586(3) 20.0544(4)

C58H59Cl4MnN2OP4 1120.69 120(2) orthorhombic Pna21 28.894(5) 13.643(2) 13.733(2)

92.1980(10) 9684.0(3) 8 3.805 1.54178 1.328 0.35  0.23  0.11 2.44 to 67.69 21 842 8367 [R(int) = 0.0363]

5413.6(15) 4 0.602 0.71073 1.375 0.18  0.18  0.06 1.41 to 33.27 59 506 18 255 [R(int) = 0.0966]

2191 R1 = 0.0643, wR2 = 0.1301 R1 = 0.1363, wR2 = 0.1473 1.010

)

)

650 587 635 R1 = 0.0506, R1 = 0.0443, R1 = 0.0564, wR2 = 0.1089 wR2 = 0.1187 wR2 = 0.1089 R1 = 0.0963, R1 = 0.0509, R1 = 0.0957, wR2 = 0.1249 wR2 = 0.1241 wR2 = 0.1260 1.045 1.009 goodness-of-fit on F2 1.019 absorption corr semiempirical from equivalents P P P P a R1 = Fo| - |Fc / |Fo|; wR2 = { [w(|Fo2| - |Fc2|)2]/ [w|Fo2|2]}1/2.

C176H127B2F49Mn2N2O2P8 3612.06 100(2) triclinic P1 16.8661(7) 22.7560(9) 24.8001(10) 64.6560(10) 89.5880(10) 80.3940(10) 8459.6(6) 2 0.333 0.71073 1.418 0.23  0.22  0.10 1.51 to 33.22 149 486 64 408 [R(int) = 0.0551]

butyl CH2); 0.74 (t, JHH = 7.3, 3H, butyl CH3). 31P{1H} NMR (CD2Cl2): δ 42.0 (s, dppm); 14.4 (s, PNP). Mn(dppp)(dppm)(CO)(Br). Yield: 30%, as a pale orange solid. 1 H NMR (benzene-d6): δ 7.81 (br s, 4H, aromatic); 6.78-7.45 (36H, aromatic); 4.36 (dt, JHH = 14.4 Hz, JPH = 9.3 Hz, 1H, dppm CH2); 4.01 (m, 1H, dppm CH2); 3.63 (t, JHH = 10.6 Hz, 2H, dppp CH2); 2.00 (br s, 2H, dppp CH2); 1.63 (m, 1H, dppp CH2); 1.31 (m, 1H, dppp CH2). 31P{1H} NMR (benzene-d6): δ 47.7 (s, dppm); 19.3 (s, PNP). Mn(PPh2NBn2)(dppm)(CO)(Br). A solution of Mn(PPh2NBn2)(CO)3(Br) (525 mg, 0.738 mmol) and dppm (423 g, 1.10 mmol) in fluorobenzene (45 mL) was stirred and photolyzed under a stream of N2. The reaction was monitored by 31P NMR for disappearance of the starting material signal (∼4 to 6 h). The resulting red solution was evaporated under vacuum to give a red solid. The solids were dissolved in minimal THF (∼5 mL). Hexane (20 mL) was then added to give an oil, which was stirred overnight in the THF/hexane mixture to yield a powder. A red solid was collected by filtration and dried under vacuum. Yield: 520 mg (0.505 mmol, 67%). 1H NMR (CD2Cl2): δ 6.75-7.50 (40H, aromatic); 4.71 (m, 1H, dppm CH2); 4.68 (m, 1H, dppm CH2); 4.60 (dt, JHH = 10.8 Hz, JPH = 3.0 Hz, 2H, P2N2 CH2); 3.95 (s, 2H, benzyl CH2); 3.79 (s, 2H, benzyl CH2); 3.35 (dt, JHH = 10.8 Hz, JPH = 3.0 Hz, 2H, P2N2 CH2); 3.17 (d, JHH = 12.6 Hz, 2H, P2N2 CH2); 2.91 (dt, JHH = 10.8 Hz, JPH = 6.0 Hz, 2H, P2N2 CH2). 31P{1H} NMR (CD2Cl2): δ 49.2 (s, dppm): 29.8 (s, P2N2). IR: νCO (CH2Cl2) = 1835(s) cm-1. Mn(dppe)2(CO)(H). LiAlH4 (34 mg, 0.90 mmol) was added to a solution of Mn(dppe)2(CO)(Br) (198 mg, 0.206 mmol) in THF (3.10 g), and the mixture was stirred overnight (∼16 h). The resulting red-brown mixture was filtered through Celite to give an orange solution. Water (∼0.1 mL) was added until the color changed to yellow and no more bubbling occurred upon water addition. The resulting mixture was filtered through Celite to remove all solids. The solvent was removed under vacuum to give a yellow solid. Yield: 132 mg (0.150 mmol, 73%). 1H NMR (CD2Cl2): δ 7.42 (t, JPH = 6 Hz, 8H, aromatic); 6.98-7.20 (32H, aromatic); 2.26 (m, 4H, CH2); 1.94 (m, 4H, CH2); -7.32 (p, JPH =

44 Hz, 1H, hydride). 31P{1H} NMR (CD2Cl2): δ 99.5 (s). IR: νCO (CH2Cl2) = 1814(s) cm-1. Anal. Calc for C53H49MnOP4: C, 70.27; H, 5.61. Found: C, 71.06; H, 5.30. Mn(PNMeP)(dppm)(CO)(H). LiAlH4 (121 mg, 3.19 mmol) was added to a stirred solution of Mn(PNMeP)(dppm)(CO)(Br) (303.7 mg, 0.312 mmol) in THF (20 mL). After stirring overnight (∼16 h), the reaction mixture was red-brown. The reaction was filtered through Celite to give a yellow solution. Water (∼0.1 mL) was added dropwise to the solution until no more bubbling occurred upon addition. The resulting yellow mixture was filtered through Celite to remove solids, and the solution was evaporated to dryness to give a yellow solid. Yield: 253 mg (0.282 mmol, 90%). 1H NMR (CD2Cl2): δ 7.44 (m, 8H, aromatic); 6.90-7.20 (32H, aromatic); 4.32 (dt, JHH = 13.8 Hz, JPH = 10.5 Hz, 1H, dppm CH2); 3.71 (m, JHH = 13.8, 7.5 Hz, 1H, dppm CH2); 3.30 (dt, JHH = 12.7 Hz, JPH = 6.3 Hz, 2H, PNP CH2); 3.21 (dt, JHH = 12.7 Hz, JPH = 3.7 Hz, 2H, PNP CH2); 2.19 (s, 3H, CH3); -3.58 (ttd, JPH = 49.3, 34.2 Hz, JHH = 7.5 Hz, 1H, hydride). 31P{1H} NMR (CD2Cl2): δ 65.6 (d, JPP = 37.3 Hz, dppm); 44.2 (d, JPP = 37.3 Hz, PNP). IR: νCO (CH2Cl2) = 1797(s) cm-1. Anal. Calc for C53H50BrMnNOP2: C, 71.06; H, 5.63; N, 1.56. Found: C, 70.44; H, 5.81; N, 1.65. Mn(PPh2NBn2)(dppm)(CO)(H). LiAlH4 (138 mg, 3.64 mmol) was added to a stirred solution of Mn(PPh2NBn2)(dppm)(CO)(Br) (290 mg, 0.282 mmol) in THF (10 g). After stirring overnight (∼16 h), the brown mixture was filtered through Celite to give an orange solution. Water (∼0.1 mL) was added dropwise until the reaction mixture was yellow and bubbling no longer occurred upon water addition. Solids were removed by filtration through Celite, and the solution was evaporated to dryness to give a yellow solid. Yield: 269 mg (0.281 mmol, 99%). 1H NMR (CD2Cl2): δ 6.75-7.65 (40H, aromatic); 4.15 (s, 2H, benzyl CH2); 4.07 (ddt, JHH = 13.9, 7.3 Hz, JPH = 7.6 Hz, 1H, dppm CH2); 3.92 (dt, JHH = 13.9 Hz, JPH = 10.3 Hz, 1H, dppm CH2); 3.60 (s, 2H, benzyl CH2); 3.07 (d, JHH = 11.2 Hz, 2H, P2N2 CH2); 3.04 (d, JHH = 11.2 Hz, 2H, P2N2 CH2); 2.84 (d, JHH = 11.8 Hz, 2H, P2N2 CH2); 2.64 (dt, JHH = 11.8 Hz, JPH = 4.3 Hz, 2H, P2N2 CH2); -5.11 (ttd, JPH = 46.4, 38.3 Hz, JHH = 7.3, 1H,

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Organometallics, Vol. 29, No. 20, 2010

hydride). 31P{1H} NMR (CD2Cl2): δ 67.8 (dd, JPP = 30.7, 10.5 Hz, dppm); 53.5 (dd, JPP = 30.7, 10.5 Hz, P2N2). IR: νCO (CH2Cl2) = 1811(s) cm-1. In Situ Formation of Cation Complexes. [Mn(PNMeP)(dppm)(CO)][BArF4]. A J. Young NMR tube was charged with Mn(PNMeP)(dppm)(CO)(Br) (14.5 mg, 0.0149 mmol) and NaBArF4 (23.1 mg, 0.0261 mmol) in a glovebox under argon. The tube was evacuated on a high vacuum line, and fluorobenzene (∼0.5 mL) was vacuum transferred from P2O5. The tube was sealed. Upon shaking a dark blue solution and a white precipitate formed. 31 P{1H} NMR (C6H5F): δ 52.4 (s, dppm), 30.3 (s, PNP). IR: νCO (C6H5F) = 1841(s) cm-1. [Mn(PNPhP)(dppm)(CO)][BArF4]. Prepared in an analogous fashion to [Mn(PNMeP)(dppm)(CO)][BArF4] from Mn(PNPhP)(dppm)(CO)(Br), generating a dark blue solution. 31P{1H} NMR (C6H5F): δ 54.5 (s, dppm), 30.3 (s, PNP). IR: νCO (C6H5F) = 1842(s) cm-1. [Mn(PNBuP)(dppm)(CO)][BArF4]. Prepared in an analogous fashion to [Mn(PNMeP)(dppm)(CO)][BArF4] from Mn(PNBuP)(dppm)(CO)(Br), generating a dark blue solution. 31P{1H} NMR (C6H5F): δ 53.5 (s, dppm), 30.7 (s, PNP). IR: νCO (C6H5F) = 1840(s) cm-1. [Mn(dppp)(dppm)(CO)][BArF4]. Prepared in an analogous fashion to [Mn(PNMeP)(dppm)(CO)][BArF4] from Mn(dppp)(dppm)(CO)(Br), generating a dark blue solution. 31P{1H} NMR (C6H5F): δ 53.4 (s, dppm), 29.4 (s, PNP). IR: νCO (C6H5F) = 1839(s) cm-1. [Mn(P2PhN2Bn)(dppm)(CO)][BArF4]. Prepared in an analogous fashion to [Mn(PNMeP)(dppm)(CO)][BArF4] from Mn(P2PhN2Bn)(dppm)(CO)(Br), generating a red solution. 31P{1H} NMR (C6H5F): δ 24.6 (s), 23.7 (s). IR: νCO (C6H5F) = 1843(s) cm-1. Formation of H2 and N2 Complexes. In a representative experiment, a J. Young NMR tube was charged with 14.8 mg (0.0152 mmol) of the bromide Mn(PNMeP)(dppm)(CO)(Br) and 18.9 mg (0.0213 mmol, 1.4 equiv) of NaBArF4 under an argon atmosphere. The tube was evacuated, and 0.53 mL of fluorobenzene was vacuum transferred into the tube. The tube was

Welch et al. allowed to warm to room temperature and shaken to give a dark blue solution and a white precipitate. The solution was frozen in liquid nitrogen, and maintaining liquid nitrogen temperatures, 1 atm of hydrogen gas was introduced into the tube from a 2 L ballast. The tube was sealed and allowed to warm to room temperature. Upon shaking, the color of the solution changed from blue to yellow. The procedure was repeated for each bromide and for both H2 and N2. Spectral data are provided in Table 2. Determination of Equilibrium Constants for Binding of H2 and N2. In a representative reaction, a J. Young NMR tube was charged with 20.1 mg (0.0206 mmol) of the bromide Mn(PNMeP)(dppm)(CO)(Br) and 25.9 mg (0.0292 mmol, 1.4 equivalents) of NaBArF4 under an argon atmosphere. The tube was evacuated, and 0.57 mL of fluorobenzene was vacuum transferred into the tube. Upon warming to room temperature and mixing, a dark blue solution and white precipitate formed. Then 102 Torr of hydrogen gas was transferred into the tube, and the solution was mixed for 120 s, resulting in a green solution. The tube was sealed, and the ratio of complexes was determined by 31 P NMR. The ratio was checked again after 24 h, and a second determination of the equilibrium value was determined starting from the H2 complex, to ensure equilibrium was achieved. This procedure was repeated for each bromide for both H2 and N2 coordination.

Acknowledgment. We thank the Division of Chemical Sciences, Biosciences and Geosciences, Office of Basic Energy Sciences, Office of Science of the U.S. Department of Energy, for support of this work. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy. Supporting Information Available: 1H and 31P NMR spectra for Mn bromides and hydrides. X-ray structure files in cif format. This material is available free of charge via the Internet at http://pubs.acs.org.