Supplementary Figure 1 IR Spectroscopy. IR spectra of 1Cu and 1Ni as well as of the starting compounds, recorded as KBr-pellets on a Bruker Alpha FTIR spectrometer. Supplementary Figure 2 UV/Vis Spectroscopy. UV/Vis spectra of 1Cu and 1Ni recorded in acetonitrile solution at room temperature on a Perkin-Elmer Lambda 2 spectrometer. 1
Supplementary Figure 3 Susceptibility temperature product as a function of temperature for 1Cu. The measurement was performed on a teflon-wrapped pressed powder sample in an applied field of 1000 Oe with a Quantum Design MPMS XL7 SQUID magnetometer. The data were corrected for the diamagnetic contribution by using Pascal's constants. 1,0 Brillouin functions 1.8 K 2 K 3 K 5 K 7 K 10 K M / B 0,5 0,0 0 10 20 30 40 50 60 70 H / koe Supplementary Figure 4 Magnetization as a function of field for 1Cu. The measurement was performed on a teflon-wrapped pressed powder sample at various temperatures as indicated in the Figure with a Quantum Design MPMS XL7 SQUID magnetometer. The data were corrected for the diamagnetic contribution by using Pascal's constants. Solid red lines are fits to the Brillouin function with S = ½ and g = 2.048. 2
a b Intensity (arb. u.) Intensity (arb. u.) 300 320 340 360 300 320 340 360 Magnetic field (mt) Supplementary Figure 5 CW X-band EPR spectra of 1Cu 1.5%. Experimental CW X-Band EPR spectra (blue solid lines) and easy spin simulations (red dashed lines). The spectra were recorded on a finely ground powder sample of 1Cu 1.5% in evacuated sample tubes at 9.47 GHz on a Bruker EMX spectrometer in Stuttgart. a, Spectrum at 5 K and fit with parameters g = 2.0932 ± 0.002, g = 2.0240 ± 0.001, A = 496 ± 5 MHz, A = 119 ± 5 MHz. b, Spectrum at room temperature and fit with parameters g = 2.0910 ± 0.002, g = 2.0238 ± 0.001, A = 480 ± 5 MHz, A = 115 ± 5 MHz. 3
a b 1120 1160 1200 1120 1160 1200 c d Intensity (arb. u.) e 1120 1160 1200 f 1120 1160 1200 1120 1160 1200 Magnetic field (mt) 1120 1160 1200 Magnetic field (mt) Supplementary Figure 6 Pulsed Q-band EPR spectra of 1Cu 0.001%. Electron spin echo detected field-swept spectra recorded on 1Cu 0.001% (blue solid lines) and easy spin simulations (red dashed lines) using the parameters g = 2.0925 ± 0.010, g = 2.0227 ± 0.005, A = 500 ± 5 MHz, A = 118 ± 5 MHz. The measurements were performed on a Bruker Elexsys E580 in Frankfurt. Temperatures up to 275 K were achieved with an Oxford Instruments CF935 continuous flow cryostat (cryogenic liquid: He), room temperature measurements were done at approximately 294 K. a, Spectrum recorded at 7 K and 33.77 GHz. b, Spectrum recorded at 15 K and 33.77 GHz. c, Spectrum recorded at 50 K and 33.8 GHz. d, Spectrum recorded at 120 K and 33.77 GHz. e, Spectrum recorded at 200 K and 33.73 GHz. f, Spectrum recorded at room temperature and 33.53 GHz. 4
Supplementary Figure 7 Hahn echo decay curves of 1Cu 0.001%. Normalized echo intensity as a function of delay time 2τ (Hahn echo sequence) recorded on a powder sample of 1Cu 0.001% at different temperatures. Black solid lines are stretched exponential fits resulting in the coherence times reported in Supplementary Table 1. The measurements were performed at 34 GHz on a Bruker Elexsys E580 in Frankfurt. Temperatures up to 275 K were achieved with an Oxford Instruments CF935 continuous flow cryostat (cryogenic liquid: He), room temperature measurements were done at approximately 294 K. 5
Intensity / arb. u. 0,3 0,2 0,1 0,0 1,16 1,20 1,24 1,28 Magnetic Field / T a / arb. units I*I 1 0 I / arb. units 1,0 0,8 0,6 0,4 0,2 0,0 1 0-1 b 0 5 10 2 / s 15 c 0 100 200 300 400 T / ms Supplementary Figure 8 Pulsed EPR on 1Cu 1.5%. The measurements were performed at 35 GHz on a homebuilt pulsed Q-band EPR spectrometer in Stuttgart. a, echo detected EPR spectrum recorded at 7 K; b, Hahn echo decay curve at 7 K together with stretched exponential fit, where T M = 3.97 μs, and x = 1.43; c, inversion recovery curve together with a biexponential fit at 7 K, where T 1,f = 2.5 ms and T 1,s = 41 ms. 6
Supplementary Figure 9 Nutation measurement of 1Cu 0.001%. Nutation measurement on a powder sample of 1Cu 0.001% at 15 K and different applied powers were performed at 34 GHz on a Bruker Elexsys E580 in Frankfurt. Pulse sequence: nutation pulse - τ nut - π/2 - τ fix - π - τ fix - echo with τ nut = 400 ns, τ fix = 140 ns. 7
Supplementary Figure 10 Inversion recovery traces of 1Cu 0.001%. Normalized echo intensity as a function of delay time T (inversion recovery sequence) at different temperatures. Black solid lines are biexponential (for 7-25 K) or monoexponential (for T > 25 K) fit functions resulting in the spin lattice relaxation times reported in Supplementary Table 2. The measurements were performed at 34 GHz on a Bruker Elexsys E580 in Frankfurt. Temperatures up to 275 K were achieved with an Oxford Instruments CF935 continuous flow cryostat (cryogenic liquid: He), room temperature measurements were done at approximately 294 K. 8
Intensity / arb. u. 0,05 0,04 0,03 0,02 0,01 0,00 1,16 1,20 1,24 1,28 Magnetic Field / mt a Echo Intensity / arb. u. 1,0 7 K b 50 K 150 K 0,1 0 40 80 120 2 / s 1,5 1,0 0,5 0,0-0,5-1,0 1 10 100 1000 T / s c Supplementary Figure 11 Pulsed EPR on 1Cu 0.01% D. The measurements were performed at 35 GHz on a homebuilt pulsed Q-band EPR spectrometer in Stuttgart. a, echo detected EPR spectrum recorded at 7 K; b, Hahn echo decay curves together with fit functions at different temperatures; c, inversion recovery curves together with fit functions at different temperatures. Fit functions are bi- (7 K) or monoexponentials (50, 100 K) for both relaxation times and their results are reported in Supplementary Table 3. 9
Supplementary Table 1 Coherence times T M and stretch factors for 1Cu 0.001%. Parameters were extracted from fitting Hahn echo decay curves of Supplementary Figure 7 with stretched exponentials. T / K T M / μs x 7 9.229 ± 0.010 2.480 ± 0.010 8 9.137 ± 0.011 2.408 ± 0.011 9 9.163 ± 0.010 2.443 ± 0.009 10 9.195 ± 0.010 2.468 ± 0.010 12 9.145 ± 0.011 2.453 ± 0.010 15 9.128 ± 0.011 2.445 ± 0.011 17 9.079 ± 0.014 2.351 ± 0.012 20 9.032 ± 0.014 2.390 ± 0.013 25 8.950 ± 0.015 2.350 ± 0.014 50 8.301 ± 0.022 2.099 ± 0.017 75 7.458 ± 0.025 1.828 ± 0.016 100 6.951 ± 0.023 1.709 ± 0.013 120 5.804 ± 0.017 1.457 ± 0.008 150 4.869 ± 0.019 1.401 ± 0.010 175 3.463 ± 0.014 1.181 ± 0.007 200 2.528 ± 0.017 1.045 ± 0.008 225 1.679 ± 0.010 1.027 ± 0.007 250 1.168 ± 0.013 0.892 ± 0.010 275 1.121 ± 0.003 1.173 ± 0.004 rt 0.600 ± 0.002 1.379 ± 0.006 Supplementary Table 2 Spin lattice relaxation times T 1 (fast and slow components) for Cu 0.001%. Parameters were extracted from fitting Inversion recovery traces of Supplementary Figure 10 with biexponential (for 7-25 K) or monoexponential (for T > 25 K) fit functions. T / K T 1,f / μs T 1,s / μs 7 6852 195 87380 209 15 358 17 3707 7 25 58 4 592 1 50-69.1 0.1 75-30.32 0.08 100-19.79 0.05 120-9.51 0.02 150-5.74 0.02 175-3.67 0.01 200-2.520 0.007 225-1.291 0.005 250-0.910 0.008 275-0.692 0.002 rt - 0.477 0.002 10
Supplementary Table 3 Spin lattice relaxation times T 1 and spin-spin relaxation times T M (fast and slow components in both cases) for Cu 0.01% D. Parameters were extracted from fitting Inversion recovery traces and Hahn echo decays shown in Supplementary Figure 11 with biexponential (for 7 K) or monoexponential (for 50, 100 K) fit functions. T / K T 1,f / μs T 1,s / μs T M,f / μs T M,s / μs 7 7 ± 3 96197 ± 9356 4.2 ± 0.3 68 ± 3 50-83.5 ± 0.7-19.8 ± 0.1 150-1.60 ± 0.05-4.2 ± 0.2 Supplemetary Methods Synthesis and Analysis All reagents and solvents were used as purchased from commercial sources. Bis-(tetraphenylphosphonium)-bis-(maleonitriledithiolato)cuprate, (PPh 4 ) 2 [Cu(mnt) 2 ] (1Cu): Sodium maleonitriledithiolate (279 mg, 1.5 mmol) was dissolved in 5 ml ethanol and 2 ml demineralized water. Subsequently copper chloride dihydrate (128 mg, 0.75 mmol), dissolved in 5 ml ethanol, and tetraphenylphosphonium bromide (629 mg, 1.50 mmol), dissolved in 15 ml ethanol were added under stirring. The brown product precipitated immediately and was separated from the solution after 5 min by vacuum filtration. Washing of the product with 3 x 5 ml ethanol and drying for 20 h under reduced pressure gave a yield of 576 mg (75.01 % based on Cu). Elemental analysis: found (calcd) for C 59 H 40 CuN 4 P 2 S 4 in %: C: 65.42 (65.77), H: 3.90 (3.94), N: 5.48 (5.48), S: 12.70 (12.54). UV/VIS (MeCN): λ max in nm: 478, 430, 369, 348, 317, 275. IR (KBr-pellet): in cm -1 and assignment in brackets: 2195 (ν C N ), 1459 (ν C=C ). Bis-(tetraphenylphosphonium)-bis-(maleonitriledithiolato)nickelate (PPh 4 ) 2 [Ni(mnt) 2 ] (1Ni): The same procedure as described for 1Cu was executed with nickel chloride hexahydrate (178 mg, 0.75 mmol) instead of copper chloride dihydrate. The red compound was isolated with a yield of 614 mg (80.4 % based on Ni). Elemental analysis: found (calcd) for C 59 H 40 NiN 4 P 2 S 4 : C: 65.75 (66.08), H: 3.94 (3.96), N: 5.46 (5.50), S: 12.83 (12.60). UV/VIS (MeCN): λ max in nm: 474, 379, 315, 270. IR (KBr-pellet): in cm -1 and assignment in brackets: 2196 (ν C N ), 1479 (ν C=C ). Perdeuterotetraphenylphosphonium bromide PPh 4 Br -d 20 : In a pressure cylinder, deuterated triphenylphosphine (1.0000 g, 3.6 mmol) was combined with anhydrous nickel(ii)bromide (0.3933 g, 1.8 mmol) and deuterated phenylbromide (0.76 ml, 7.2 mmol). The pressure cylinder was sealed and heated to 180 C for three hours. After cooling, 50 ml of demineralized water was added to the blue-green solid. After stirring for 15 min at 75 C, a watery blue solution and a fawn precipitate formed. The mixture was cooled in an ice bath and extracted with 3 x 50 ml of diethylether. The organic phase of the ether extraction was discarded and the aqueous phase was extracted with 3 x 50 ml chloroform. Subsequently, the solvent of the organic phase was removed and the resulting white solid was dried under reduced pressure. 13 C NMR (500MHz): 135.41 (t), 133.96 (td), 130.82 (td), 117.56, 116.85 ppm. 11
Bis-(perdeuterotetraphenylphosphonium)-bis-(maleonitriledithiolato)cuprate (PPh 4 -d 20 ) 2 [Cu(mnt) 2 ] (1CuD): The deuterated compound was synthesized following the same procedure as described for its protonated analogue, except that PPh 4 Br was replaced by d 20 -PPh 4 Br. UV/VIS in DCM: λ max in nm: 482, 388, 373, 328, 323, 287, 276, 268. IR (KBrpellet): in cm -1 and assignment in brackets: 2195 (ν C N ), 1459 (ν C=C ). Bis-(perdeuterotetraphenylphosphonium)-bis-(maleonitriledithiolato)nickelate (PPh 4 -d 20 ) 2 [Ni(mnt) 2 ] (1NiD): The deuterated complex was synthesized following the same procedure as given for its protonated analogue above, where PPh 4 Br was replaced by d 20 - PPh 4 Br. UV/VIS in DCM: λ max in nm: 481, 384, 318, 272, 229. IR (KBr-pellet): in cm -1 and assignment in brackets: 2195 (ν C N ), 1479 (ν C=C ). Doped powder: 0.001 % of 1Cu in 1Ni (1Cu 0.001% ): Doped powders 1Cu 0.001% were obtained by dissolving compounds 1Cu and 1Ni in the molar ratio 0.001 : 99.999 in a minimum volume of acetone, which was subsequently evaporated under reduced pressure. The resulting powders were dried in vacuo and finely ground. Doped powder: 0.01 % of 1CuD in 1NiD (1Cu 0.01% D): Doped powders 1Cu 0.01% D wereobtained similarly to 1Cu 0.001% from 1CuD and 1NiD. Doped single crystal: 1.5 % of 1Cu in 1Ni (1Cu 1.5% ): X-ray quality single crystals were obtained by dissolving compounds 1Cu and 1Ni in the molar ratio 98.5 : 1.5 in a minimum volume of acetone, which was subsequently diluted with the same amount of methanol. Crystals were collected after one week of slow solvent evaporation under ambient conditions. Results of X-ray analysis of 1Cu 1.5% : monoclinic, a = 11.3221(10), b = 15.0460(13), c = 13.9864(12) Å, β = 98.201(5), V = 2358.3(4) Å 3, T = 110 K, space group P2 1 /n, Z = 4, 32268 reflections collected, 7183 unique, final R 1 = 0.0514, wr 2 [I > 2σ(I)] = 0.1275, GOF = 1.034. 12