Natural killer cell functional genetics and donor-specific antibody-triggered microvascular inflammation.

Antibody-mediated rejection (ABMR) is a leading cause of graft failure. Emerging evidence suggests a significant contribution of natural killer (NK) cells to microvascular inflammation (MVI). We investigated the influence of genetically determined NK cell functionality on ABMR development and activity. The study included 86 kidney transplant recipients subjected to systematic biopsies triggered by donor-specific antibody detection. We performed killer immunoglobulin-like receptor typing to predict missing self and genotyped polymorphisms determining NK cell functionality (FCGR3AV/F158 [rs396991], KLRC2wt/del, KLRK1HNK/LNK [rs1049174], rs9916629-C/T). Fifty patients had ABMR with considerable MVI and elevated NK cell transcripts. Missing self was not related to MVI. Only KLRC2wt/wt showed an association (MVI score: 2 [median; interquartile range: 0-3] vs 0 [0-1] in KLRC2wt/del recipients; P = .001) and remained significant in a proportional odds multivariable model (odds ratio, 7.84; 95% confidence interval, 2.37-30.47; P = .001). A sum score incorporating all polymorphisms and missing self did not outperform a score including only KLRC2 and FCGR3A variants, which were predictive in univariable analysis. NK cell genetics did not affect graft functional decline and survival. In conclusion, a functional KLRC2 polymorphism emerged as an independent determinant of ABMR activity, without a considerable contribution of missing self and other NK cell gene polymorphisms.

Randomized Trial to Assess the Clinical Utility of Renal Allograft Monitoring by Urine CXCL10 Chemokine.

SIGNIFICANCE STATEMENT: This study is the first randomized controlled trial to investigate the clinical utility of a noninvasive monitoring biomarker in renal transplantation. Although urine CXCL10 monitoring could not demonstrate a beneficial effect on 1-year outcomes, the study is a rich source for future design of trials aiming to explore the clinical utility of noninvasive biomarkers. In addition, the study supports the use of urine CXCL10 to assess the inflammatory status of the renal allograft. BACKGROUND: Urine CXCL10 is a promising noninvasive biomarker for detection of renal allograft rejection. The aim of this study was to investigate the clinical utility of renal allograft monitoring by urine CXCL10 in a randomized trial. METHODS: We stratified 241 patients, 120 into an intervention and 121 into a control arm. In both arms, urine CXCL10 levels were monitored at three specific time points (1, 3, and 6 months post-transplant). In the intervention arm, elevated values triggered performance of an allograft biopsy with therapeutic adaptations according to the result. In the control arm, urine CXCL10 was measured, but the results concealed. The primary outcome was a combined end point at 1-year post-transplant (death-censored graft loss, clinical rejection between month 1 and 1-year, acute rejection in 1-year surveillance biopsy, chronic active T-cell-mediated rejection in 1-year surveillance biopsy, development of de novo donor-specific HLA antibodies, or eGFR <25 ml/min). RESULTS: The incidence of the primary outcome was not different between the intervention and the control arm (51% versus 49%; relative risk (RR), 1.04 [95% confidence interval, 0.81 to 1.34]; P = 0.80). When including 175 of 241 (73%) patients in a per-protocol analysis, the incidence of the primary outcome was also not different (55% versus 49%; RR, 1.11 [95% confidence interval, 0.84 to 1.47]; P = 0.54). The incidence of the individual end points was not different as well. CONCLUSIONS: This study could not demonstrate a beneficial effect of urine CXCL10 monitoring on 1-year outcomes (ClinicalTrials.gov_ NCT03140514 ).

Urine CXCL10 as a biomarker in kidney transplantation.

PURPOSE OF REVIEW: Urine CXCL10 is a promising biomarker for posttransplant renal allograft monitoring but is currently not widely used for clinical management. RECENT FINDINGS: Large retrospective studies and data from a prospective randomized trial as well as a prospective cohort study demonstrate that low urine CXCL10 levels are associated with a low risk of rejection and can exclude BK polyomavirus replication with high certainty. Urine CXCL10 can either be used as part of a multiparameter based risk assessment tool, or as an individual biomarker taking relevant confounders into account. A novel Luminex-based CXCL10 assay has been validated in a multicenter study, and proved to be robust, reproducible, and accurate. SUMMARY: Urine CXCL10 is a well characterized inflammation biomarker, which can be used to guide performance of surveillance biopsies. Wide implementation into clinical practice depends on the availability of inexpensive, thoroughly validated assays with approval from regulatory authorities.

Safety and Feasibility of Immunoadsorption with Heparin Anticoagulation in Preparation of ABO-Incompatible Kidney Transplantation: A Retrospective Single-Center Study.

Introduction: Immunoadsorption (IA) of isohemagglutinins is an often-crucial procedure in preparation of major ABO blood group-incompatible living donor kidney transplantation (ABOi LDKT). Standard citrate-based anticoagulation during the procedure has potential disadvantages for distinct patient groups. In this study, we report our experience with an alternative anticoagulation scheme using heparin during IA for selected patients. Methods: We conducted a retrospective analysis of all patients who underwent IA with heparin anticoagulation between February 2013 and December 2019 at our institution with focus on the safety and efficacy of the adapted procedure. For further validation, we compared graft function, graft survival, and overall survival with those of all recipients of living donor kidney transplants with or without pretransplant desensitizing apheresis for ABO antibodies at our institution during the same period. Results: In thirteen consecutive patients prepared for ABOi LDKT with IA with heparin anticoagulation, no major bleeding or other significant complications were observed. All patients achieved sufficient isohemagglutinin titer reduction to proceed to transplant surgery. Graft function, graft survival, and overall survival did not significantly differ from patients treated with standard anticoagulation for IA or ABO compatible recipients of living donor kidneys. Conclusion: IA with heparin in preparation of ABOi LDKT is safe and feasible for selected patients after internal validation.

Impact of different urinary tract infection phenotypes within the first year post-transplant on renal allograft outcomes.

In this study, we investigated the clinical impact of different urinary tract infection (UTI) phenotypes occurring within the first year after renal transplantation. The population included 2368 transplantations having 2363 UTI events. Patients were categorized into four groups based on their compiled UTI events observed within the first year after transplantation: (i) no colonization or UTI (n = 1404; 59%), (ii) colonization only (n = 353; 15%), (iii) occasional UTI with 1-2 episodes (n = 456; 19%), and (iv) recurrent UTI with ≥3 episodes (n = 155; 7%). One-year mortality and graft loss rate were not different among the four groups, but patients with recurrent UTI had a 7-10 ml/min lower eGFR at year one (44 ml/min vs. 54, 53, and 51 ml/min; p < .001). UTI phenotypes had no impact on long-term patient survival (p = .33). However, patients with recurrent UTI demonstrated a 10% lower long-term death-censored allograft survival (p < .001). Furthermore, recurrent UTI was a strong and independent risk factor for reduced death-censored allograft survival in a multivariable analysis (HR 4.41, 95% CI 2.53-7.68, p < .001). We conclude that colonization and occasional UTI have no impact on pertinent outcomes, but recurrent UTI are associated with lower one-year eGFR and lower long-term death-censored allograft survival. Better strategies to prevent and treat recurrent UTI are needed.

Relevance of deceased donor proteinuria for kidney transplantation: A comprehensive national cohort study.

PURPOSE: Proteinuria is frequent in patients with nephropathies and associated with progressive kidney disease and risk for end stage kidney disease. However, the relevance of deceased donor proteinuria on transplant outcome remains uncertain. In this nationwide cohort study, we evaluated the prevalence of proteinuria in deceased donor candidates and measured the impact on outcome after kidney transplantation. METHODS: Data from the Swiss Organ Allocation System and the Swiss Transplant Cohort Study were analyzed, comprising 1725 donors and 1516 recipients transplanted between 2008 and 2019. We correlated urine findings with donor characteristics and quantified the impact of proteinuria on allograft function at 12 months and survival. RESULTS: Proteinuria influenced allocation decisions in 4.5% of nonimmunological organ declines and was the leading cause for decline in 0.2% of cases. 74.1%, 51.4%, and 35.3% of donor candidates had a baseline proteinuria above 15, 30, and 50 mg protein/mmol urine creatinine, respectively. Proteinuria above 30 mg/mmol was associated with female donor sex, mechanical resuscitation, acute kidney injury, and time delay between ICU entry and urine sampling. Donor proteinuria was not associated with patient or allograft survival, nor allograft function at 12 months. CONCLUSION: We report a high prevalence of proteinuria in donor candidates, without evidence of a deleterious impact of proteinuria on graft function and/or survival. Therefore, low-level proteinuria should not be considered a limiting contraindication for kidney allocation in deceased donor transplant.

Frequency, reactivity and evolution of human leukocyte antigen and human platelet antigen antibodies in the setting of hematopoietic cell transplantation.

BACKGROUND AND OBJECTIVES: Antibodies (Ab) against HLA and HPA antigens play an important role in HCT. In this prospective study we evaluated prevalence and kinetics of HLA- and HPA-Ab after HCT, including a possible donor-recipient transfer and their clinical relevance in respect to platelet transfusion refractoriness (PTR). MATERIALS AND METHODS: Patients were consecutively recruited. Ab were determined by microbead assay technique and a mean fluorescence intensity cut-off of 1,000. RESULTS: At baseline, 21 donors (42 %) and 27 patients (54 %) had HLA-Ab with a mean panel reactivity (cPRA) of 34.9 ± 29.4 % and 46.1 ± 36.5 %, respectively. We observed a significant higher number of HLA-Ab specificities in female donors and patients and a predominance of HLA-class I Ab. At day 0 we detected an increase of HLA-Ab (from 526 to 673) and cPRA (55.2 ± 31.9 %). Thirty-six patients (72 %) developed new HLA-Ab, mainly 3 weeks after HCT. In 7 patients an HLA-Ab with the same specificity as detected in the corresponding donor emerged, suggesting a possible transfer from the donor to the recipient. Overall, MFI showed a high variation. Type and number of transfusions were not associated with number and intensity of HLA-Ab (ρ: -0.05 - 0.02). Number of HLA-Ab, cPRA and intensity were not associated with PTR, which occurred in 9 patients (18 %) and none had bleeding WHO > 2. CONCLUSIONS: Although a considerable number of patients have and develop HLA-Ab before and early after HCT, we found no association with PTR and bleeding and management should be individualized.

Screening strategy for de novo donor-specific HLA antibodies beyond the first year after kidney transplantation: Personalized or "one size fits all"?

Screening for de novo donor-specific HLA antibodies (DSAs) after kidney transplantation is widely recommended. The aim of this single-center, cross-sectional study was to investigate the frequency of therapeutic interventions triggered by de novo DSA screening. We included 464 patients screened for de novo DSA at annual visits after a median of 5 years post-transplant (range 1 to 19 years). Overall, de novo DSAs were detected in 55/464 patients (11.9%) with a stepwise increase of the prevalence from 4.9% at 1 year post-transplant to 18.9% at >10 years post-transplant. Subsequent allograft biopsies were performed in 24/55 patients (44%). The main reasons to omit biopsies were good/stable allograft function and anticipated lack of clinical consequences (eg, relevant comorbidities). Rejection processes were detected in 16/24 biopsies (67%). Therapeutic interventions were made in 18/464 screened patients (3.9%) with a significantly higher rate in the youngest quartile of patients (≤48 years; 7.9%) compared to the middle 50% (49-67 years; 3%) and the oldest quartile (≥68 years; 1.7%) (P = .03). Our study suggests that the frequency of therapeutic interventions triggered by de novo DSA screening after kidney transplantation is overall low, but significantly higher in younger patients, arguing for a personalized, age-adapted de novo DSA screening strategy.

Short- and long-term impact of neutropenia within the first year after kidney transplantation.

The aim of this retrospective single-center study was to investigate the short- and long-term impact of neutropenia occurring within the first year after kidney transplantation, with a special emphasis on different neutropenia grades. In this unselected cohort, 225/721 patients (31%) developed 357 neutropenic episodes within the first year post-transplant. Based on the nadir neutrophil count, patients were grouped as neutropenia grade 2 (<1.5-1.0*109 /l; n = 105), grade 3 (<1.0-0.5*109 /l; n = 65), and grade 4 (<0.5*109 /l; n = 55). Most neutropenia episodes were presumably drug-related (71%) and managed by reduction/discontinuation of potentially responsible drugs (mycophenolic acid [MPA] 51%, valganciclovir 25%, trimethoprim/sulfamethoxazole 19%). Steroids were added/increased as replacement for reduced/discontinued MPA. Granulocyte colony-stimulating factor was only used in 2/357 neutropenia episodes (0.6%). One-year incidence of (sub)clinical rejection, one-year mortality, and long-term patient and graft survival were not different among patients without neutropenia and neutropenia grade 2/3/4. However, the incidence of infections was about 3-times higher during neutropenia grade 3 and 4, but not increased during grade 2. In conclusion, neutropenia within the first year after kidney transplantation represents no increased risk for rejection and has no negative impact on long-term patient and graft survival. Adding/increasing steroids as replacement for reduced/discontinued MPA might supplement management of neutropenia.

Pre-transplant donor-specific HLA antibodies and risk for poor first-year renal transplant outcomes: results from the Swiss Transplant Cohort Study.

The aim of this study was to analyze first year renal outcomes in a nationwide prospective multicenter cohort comprising 2215 renal transplants, with a special emphasis on the presence of pre-transplant donor-specific HLA antibodies (DSA). All transplants had a complete virtual crossmatch and DSA were detected in 19% (411/2215). The investigated composite endpoint was a poor first-year outcome defined as (i) allograft failure or (ii) death or (iii) poor allograft function (eGFR ≤25 ml/min/1.73 m2 ) at one year. Two hundred and twenty-one (221/2215; 10%) transplants showed a poor first-year outcome. Rejection (24/70; 34%) was the most common reason for graft failure. First-year patient's death was rare (48/2215; 2%). There were no statistically significant differences between DSA-positive and DSA-negative transplants regarding composite and each individual endpoint, as well as reasons for graft failure and death. DSA-positive transplants experienced more frequently rejection episodes, mainly antibody-mediated rejection (both P < 0.0001). The combination of DSA and any first year rejection was associated with the overall poorest death-censored allograft survival (P < 0.0001). In conclusion, presence of pre-transplant DSA per se does not affect first year outcomes. However, DSA-positive transplants experiencing first year rejection are a high-risk population for poor allograft survival and may benefit from intense clinical surveillance.

Outcome of kidney transplantation from very senior donors in Switzerland - a national cohort study.

Kidney transplantation from older and marginal donors is effective to confront organ shortage. However, limitations after transplantation of kidneys from very marginal kidney donors remain unclear. We compared patient and graft outcome, achieved allograft function and quality of life of renal transplantations from Very Senior Donors (VSD, defined as donors aged 70 years and older) with Senior Donors (SD, aged 60-70 years) and Regular Donors (RD, aged younger than 60 years) in Switzerland. We evaluated the outcome of 1554 adult recipients of deceased donor kidney transplantations from 05/2008 to 12/2019; median follow-up was 4.7 years. Failure-free survival (freedom from graft loss or death), glomerular filtration rate (eGFR), and quality of life at 12 months were analyzed for RD (reference group, n = 940), SD (n = 404), and VSD (n = 210). Failure-free survival decreased with increasing donor age, mainly attributable to premature graft loss. Still, overall 5-year failure-free survival reached 83.1%, 81.0%, and 64.0% in the RD, SD, and VSD subgroups, respectively. eGFR 12 months post-transplantation was significantly higher in RD compared with SD and VSD. The acceptance rate of donor candidates for kidney TPL was 78% for the entire cohort (87% for RD, 79% for SD, and 56% for VSD). Deceased donor kidney transplantation from donors aged 70 years or older is associated with an inferior, yet acceptable failure-free outcome, with sustained quality of life.

Revisiting cytomegalovirus serostatus and replication as risk factors for inferior long-term outcomes in the current era of renal transplantation.

BACKGROUND: Cytomegalovirus (CMV) serostatus and CMV replication are considered as risk factors for inferior graft and patient survival after renal transplantation, but long-term outcome data are limited. The aim of this retrospective single-centre study was to investigate the impact of CMV serostatus and CMV replication/disease on long-term outcomes in a well-defined cohort managed by a standardized CMV prevention/treatment protocol. METHODS: We investigated 599 consecutive kidney transplantations having a CMV prevention protocol consisting of either prophylaxis (D+/R- and R+ with ATG induction) or screening/deferred therapy (R+ without ATG induction). Patients were grouped according to CMV serostatus [high risk (D+/R-): n = 122; intermediate risk (R+): n = 306; low risk (D-/R-): n = 171] and occurrence of CMV replication/disease (no CMV replication: n = 419; asymptomatic CMV replication: n = 110; CMV syndrome: n = 39; tissue-invasive CMV disease: n = 31). The median follow-up time was 6.5 years. RESULTS: Graft and patient survival were not different among the three CMV serostatus groups as well as the four CMV replication/disease groups (P ≥ 0.44). Eighty-seven patients died, 17 due to infections (21%), but none was attributable to CMV. The overall hospitalization incidence for CMV-related infection was 3% (17/599 patients). The incidence of clinical and (sub)clinical rejection was similar among the groups (P ≥ 0.17). In a multivariate Cox proportional hazard model, neither CMV serostatus, nor CMV replication, nor CMV disease were independent predictors for patient death or graft failure, respectively. CONCLUSIONS: This retrospective single-centre study suggests that the negative impact of CMV infection on long-term patient and allograft survival as well as on allograft rejection can be largely eliminated with current diagnostic/therapeutic management.

Dialysis after graft loss: a Swiss experience.

BACKGROUND: Patients returning to dialysis after graft loss have high early morbidity and mortality. METHODS: We used data from the Swiss Transplant Cohort Study to describe the current practice and outcomes in Switzerland. All patients who received a renal allograft between May 2008 and December 2014 were included. The patients with graft loss were divided into two groups depending on whether the graft loss occurred within 1 year after transplantation (early graft loss group) or later (late graft loss group). Patients with primary non-function who never gained graft function were excluded. RESULTS: Seventy-seven out of 1502 patients lost their graft during follow-up, 40 within 1 year after transplantation. Eleven patients died within 30 days after allograft loss. Patient survival was 86, 81 and 74% at 30, 90 and 365 days after graft loss, respectively. About 92% started haemodialysis, 62% with definitive vascular access, which was associated with decreased mortality (hazard ratio = 0.28). At the time of graft loss, most patients were on triple immunosuppressive therapy with significant reduction after nephrectomy. One year after graft loss, 77.5% (31 of 40) of patients in the early and 43.2% (16 out of 37) in the late-loss group had undergone nephrectomy. Three years after graft loss, 36% of the patients with early and 12% with late graft loss received another allograft. CONCLUSION: In summary, our data illustrate high mortality, and a high number of allograft nephrectomies and re-transplantations. Patients commencing haemodialysis with a catheter had significantly higher mortality than patients with definitive access. The role of immunosuppression reduction and allograft nephrectomy as interdependent factors for mortality and re-transplantation needs further evaluation.

Caveats of HLA antibody detection by solid-phase assays.

Solid-phase assays for human leukocyte antigens (HLA) antibody detection have clearly revolutionized the field of HLA diagnostics and transplantation. The key advantages are a high sensitivity and specificity for the detection of HLA antibodies compared with cell-based assays, as well as the potential for standardization. Solid-phase assays enabled the broad introduction of tools such as "virtual crossmatching" and "calculated panel reactive antibodies," which are essential components in many organ allocation systems, kidney-paired donation programs, and center-specific immunological risk stratification procedures. The most advanced solid-phase assays are the so-called single antigen beads (SAB). They are available now for more than 15 years, and the transplant community embraced their significant advantages. However, SAB analysis and interpretation is complex and many pitfalls have to be considered. In this review, we will discuss problems, limitations, and challenges using SAB. Furthermore, we express our wishes for the improvements of SAB as well as their future use for immunological assessment and research purposes.

Reducing calcineurin inhibitor first for treating BK polyomavirus replication after kidney transplantation: long-term outcomes.

BACKGROUND: Reducing immunosuppression is the mainstay of treating BK polyomavirus (BKPyV) viraemia after kidney transplantation, but the best approach, efficacy and impact are undefined. We established a standard operating procedure (SOP) treating BKPyV viraemia based on first reducing calcineurin inhibitor ('CNI first'). The aim of this study was to investigate long-term outcomes in 644 consecutive transplantations using this SOP. METHODS: Patients were monitored for active BKPyV infection by urinary decoy cells and, if positive, by BKPyV viraemia. In case of sustained BKPyV viraemia >1000 copies/mL, immunosuppression was reduced stepwise according to the SOP. Patients were classified as 'no decoy cells' [n = 432 (66%)], 'decoy cells/no viraemia' [n = 107 (17%)] and 'viraemia' [n = 105 (17%)]. RESULTS: At 6-years post-transplant, graft survival was ∼84%, the clinical rejection rate was ∼25% and they were not different among the three groups (P = 0.14; P = 0.91). The median estimated glomerular filtration rate at the last follow-up was similar (range 49-53 mL/min, P = 0.08). Of 105 viraemic patients, 101 (96%) cleared BKPyV viraemia. In 39% of patients, viraemia clearance followed a tacrolimus reduction. A reduction of mycophenolic acid was required in 43% and discontinuation in 3%. No short-term graft loss was directly attributable to BKPyV-associated nephropathy. After a median follow-up of 5 years after clearance of BKPyV viraemia, 11/101 patients (11%) developed clinical rejection: 7 (7%) T-cell-mediated rejection and 4 (4%) antibody-mediated rejection (ABMR). CONCLUSIONS: Immunosuppression reduction based on 'CNI first' leads to similar long-term outcomes in patients with/without BKPyV viraemia and is associated with a low risk for ABMR after clearance of BKPyV viraemia. Randomized trials are needed to compare the risks and benefits of immunosuppression reduction strategies in kidney transplant patients with BKPyV viraemia.

The Banff Working Group Classification of Definitive Polyomavirus Nephropathy: Morphologic Definitions and Clinical Correlations.

Polyomavirus nephropathy (PVN) is a common viral infection of renal allografts, with biopsy-proven incidence of approximately 5%. A generally accepted morphologic classification of definitive PVN that groups histologic changes, reflects clinical presentation, and facilitates comparative outcome analyses is lacking. Here, we report a morphologic classification scheme for definitive PVN from the Banff Working Group on Polyomavirus Nephropathy, comprising nine transplant centers in the United States and Europe. This study represents the largest systematic analysis of definitive PVN undertaken thus far. In a retrospective fashion, clinical data were collected from 192 patients and correlated with morphologic findings from index biopsies at the time of initial PVN diagnosis. Histologic features were centrally scored according to Banff guidelines, including additional semiquantitative histologic assessment of intrarenal polyomavirus replication/load levels. In-depth statistical analyses, including mixed effects repeated measures models and logistic regression, revealed two independent histologic variables to be most significantly associated with clinical presentation: intrarenal polyomavirus load levels and Banff interstitial fibrosis ci scores. These two statistically determined histologic variables formed the basis for the definition of three PVN classes that correlated strongest with three clinical parameters: presentation at time of index biopsy, serum creatinine levels/renal function over 24 months of follow-up, and graft failure. The PVN classes 1-3 as described here can easily be recognized in routine renal biopsy specimens. We recommend using this morphologic PVN classification scheme for diagnostic communication, especially at the time of index diagnosis, and in scientific studies to improve comparative data analysis.

Successful steroid withdrawal guided by surveillance biopsies-A single-center experience.

Steroid withdrawal following renal transplantation is challenging and widely debated. This retrospective study aimed at investigating the frequency and determinants of successful steroid withdrawal guided by surveillance biopsies. We analyzed 156 pretransplant DSA-negative renal transplants receiving basiliximab induction and maintenance immunosuppression with tacrolimus-mycophenolate-steroids. The absence of rejection in surveillance biopsies at 3 or 6 months post-transplant initiated steroid withdrawal, which was monitored by subsequent indication and/or surveillance biopsies. The primary outcome was the frequency of successful (i.e., rejection-free) steroid withdrawal at 1 year post-transplant. In the whole study population, successful steroid withdrawal was achieved in 73 of 156 patients (47%). Steroid withdrawal was initiated in 98 of 156 patients (63%) and successful in 73 of 98 patients (74%). Subsequent clinical rejection occurred in only one of 98 patients (1%), whereas 24 of 98 patients (24%) experienced subclinical rejection. Steroid withdrawal was not initiated in 58 of 156 patients (37%) mainly due to current or prior severe (Banff TCMR ≥IA) subclinical rejection. Prediction of successful steroid withdrawal by pretransplant or early post-transplant parameters was poor. In conclusion, (sub)clinical rejection-free steroid withdrawal can be expected in about half of pretransplant DSA-negative patients. As successful steroid withdrawal cannot be well predicted by pre- and early post-transplant parameters, guidance by surveillance biopsies is an attractive strategy.

Synthesis of an Iron(IV) Aqua-Oxido Complex Using Ozone as an Oxidant.

The iron(IV) oxido complex [(tmc)Fe=O(OTf)]OTf with the macrocyclic ligand 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclo-tetradecane (tmc) has been synthesized using ozone as an oxidant. By adding water to this compound the complex [(H2 O)(tmc)Fe=O)](OTf)2 could be prepared. This complex is important in regard to a better understanding of the reactivity of FeIV oxido complexes. Mössbauer measurements using the solid compound showed an isomer shift of δ=0.19 mm s-1 and a quadrupole splitting ΔEQ =1.38 mm s-1 , confirming the high-valent FeIV state. DFT calculations were performed and led to an assignment of triplet spin multiplicity. Crystallographic characterization of [(H2 O)(tmc)Fe=O)](OTf)2 as well as of starting materials [(tmc)Fe(CH3 CN)](OTf)2 and [(tmc)Fe(OTf)]OTf together with previous results strongly suggest that [(H2 O)(tmc)Fe=O)](OTf)2 was formed similar to the oxido-hydroxido tautomerism analogous to heme systems.